LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

GIFT  OF" 


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NEW  EDITION  OF  STEELE'S  PHILOSOPHY. 


FOURTEEN     WEEKS 


IN 


PHYSICS 


BY 

J.  DORMAN  STEELP;  PH.D.,  F.G.S., 

AUTHOR   OF   THE    FOURTEEN- WEEKS   SERIES    IN    NATURAL   SCIENCE. 


'  The  works  of  Qod  are  fair  for  naught. 

Unless  our  eyes,  in  seeing, 
S&&  hidden  in  the  thing  the  thought 
That  animates  it* 


A.    S.     BARNES      &     COMPANY, 
NEW    YORK    AND    CHICAGO. 

(Copyright,  1869,  1878.) 


. 


A    POPULAR    SERIES 

NATURAL    SCIENCE, 

BY 

J.     I30R1MAN     STKEIvE,     F*H.D.,     K.O.S., 

A  uthor  of  the  Fourteen  Weeks  Series  in  Natural  Science,  etc.,  etc. 

New  Popular  Chemistry.  New  Descriptive  Astronomy. 

New  Popular  Physics.  New  Hygienic  Physiology. 

New  Popular  Zoology.  Popular  Geology. 

An  Introduction  to  Botany. 

The  Publishers  can  supply  (to  Teachers  only)  a  Key  containing  Answers  to  the 
Questions  and  Problems  in  Steele's  entire  Series. 


BARNES'     HISTORICAL     SERIES, 

ON     THE     PLAN     OK 

STEELE'S     FOURTEEN-WEEKS     IN     THE     SCIENCES. 

A  Brief   History  of  the   United    States. 
A   Brief   History   of  France. 

A   Brief  History  of  Ancient   Peoples. 

A   Brief   History   of   Mediaeval    and    Modern    Peoples. 
A   Brief  General    History. 

A  Brief  History  of  Greece. 

A  Brief  History  of  Rome. 

A   Popular   History  of  the   United   States. 


work  has  grown  up  in  the  class-room.  It  con- 
-L  tains  those  definitions,  illustrations,  and  applications 
which  seemed  at  the  time  to  interest  and  instruct  the 
author's  pupils.  Whenever  any  explanation  fixed  the  atten- 
tion of  the  learner,  it  was  laid  aside  for  future  use.  Thus, 
by  steady  accretions,  the  process  has  gone  on  until  a  book 
is  the  result. 

As  Physics  is  generally  the  first  branch  of  Natural 
Science  pursued  in  schools,  it  is  important  that  the 
beginner  should  not  be  wearied  by  the  abstractions  of  the 
subject,  and  so  lose  interest  in  it  at  the  very  start.  The 
author  has  therefore  endeavored  to  use  such  simple  lan- 
guage and  practical  illustrations  as  will  attract  the  learner, 
while  he  is  at  once  led  out  into  real  life.  From  the  mul- 
titude of  philosophical  principles,  only  those  have  been 
selected  which  are  essential  to  the  information  of  ever}' 
well-read  person.  Within  the  limits  of  a  small  text-book, 
no  subject  can  be  exhaustively  treated.  This  is,  however, 
of  less  importance  now,  when  every  teacher  feels  that  he 
must  of  necessity  be  above  and  beyond  any  school-work 
in  the  fulness  of  his  information.  The  object  of  an 
elementary  work  is  not  to  advance  the  peculiar  ideas  of 
any  person,  but  simply  to  state  the  currently-accepted 
facts  and  theories.  The  time-honored  classifications  recog- 
nized in  all  scientific  works,  have  been  retained.  In  order 

139140 


PREFACE. 

to  familiarize  the  pupil  with  the  metric  system,  now  gen- 
erally used  by  scientific  men,  it  is  continually  employed 
in  the  problems.  The  notes  contain  many  illustrations 
and  additional  suggestions,  but  their  great  value  will 
appear  in  the  descriptions  of  simple  experiments  which 
are  within  the  reach  of  any  pupil. 

New  plates  being  required  for  this  edition,  the  author 
has  taken  the  opportunity  thoroughly  to  revise  the  entire 
work.  By  carefully  comparing  the  criticisms  of  teachers, 
he  has  tried  to  obtain  the  "parallax"  of  all  its  statements 
and  methods,  and  to  eliminate,  as  far  as  possible,  the 
errors  growing  out  of  his  "personal  equation."  Hearty 
thanks  are  tendered  to  the  many  friends  of  the  book 
who,  by  their  suggestions  and  criticisms,  have  so  greatly 
added  to  the  value  of  this  revision.  To  name  them  all 
in  this  Preface  would  be  impossible,  and  to  discriminate 
would  be  invidious.  The  author  cannot,  however,  allow 
the  opportunity  to  pass  without  expressing  his  profound 
sense  of  obligation.  By  untiring  study  and  the  continued 
help  of  his  friends,  he  hopes  thus,  year  by  year,  to  make 
the  series  more  and  more  worthy  the  favor  which  his 
fellow-teachers  have  so  abundantly  bestowed  upon  it. 
Happy  indeed  will  he  be  if  he  succeed  in  leading  some 
young  mind  to  become  a  lover  and  an  interpreter  of 
Nature,  and  thus  come  at  last  to  see  that  Nature  herself 
is  but  a  "thought  of  God." 


TABLE    OF    CONTENTS. 


fAGH 

1,—INTRODUCTION           .  n 

MATTER ,  13 

II.— MOTION   AND   FORCE       .  25 

THREE  LAWS  OF  MOTION     ...  28 

CONSERVATION  OF  ENERGY       ...  37 

III.— ATTRACTION 41 

MOLECULAR  FORCES          .                         .  43 

GRAVITATION 52 

IV.— ELEMENTS   OF   MACHINES      .         .         .  67 

V.— PRESSURE  OF  LIQUIDS  AND  GASES  81 

HYDROSTATICS 83 

HYDRAULICS                          .        •  97 

PNEUMATICS 103 

VI.— ACOUSTICS 121 

VII.— OPTICS           .                .....  147 

VIII.— HEAT  181 


TABLE    OP 

TX.— ELECTRICITY  'A°K 

•        .      209 

X,  -APPENDIX 

2S5 
r.  QUESTIONS 

•      257 
2.  TABLES 

•        '        •          273 

3-  BLACKBOARD  DRAWINGS       •        •        .     275 

4-  INDEX 

•        •        •        •          301 


TO 


OTUDENTS  are  expected  to  obtain  information  from  this  book, 
*-}  without  the  aid  of  questions,  as  they  must  always  do  in  their 
general  reading.  When  the  subject  of  a  paragraph  is  announced,  the 
pupil  should  be  prepared  to  tell  all  he  knows  about  it.  He  should 
never  be  allowed  to  answer  a  question,  except  it  be  a  short  definition,  in 
the  language  of  the  book.  The  diagrams  and  illustrations,  as  far  as  pos- 
sible, should  be  drawn  upon  the  blackboard  and  explained.  Although 
pupils  may,  at  first,  manifest  an  unwillingness  to  do  this,  yet  in  a  little 
time  it  will  become  an  interesting  feature  of  the  recitation.  In  his  own 
classes,  the  author  has  been  accustomed  \.o  place  upon  the  blackboard  the 
analysis  of  each  chapter  of  the  book,  and  require  the  piipils  to  recite  from 
that,  without  the  interposition  of  questions,  except  such  as  were  neces- 
sary to  bring  out  the  topic  more  clearly  or  to  throw  a  side  light  upon 
it.  Where  the  analysis  given  in  the  book  does  not  include  all  the 
minor  points  of  the  lesson,  the  pupils  can  easily  supply  the  omission. 
The  "  Practical  Questions  "  given  at  the  close  of  each  general  subject 
have  been  found  a  profitable  exercise  in  awakening  inquiry  and  stimu- 
lating thought.  They  may  be  used  at  the  pleasure  of  the  instructor. 
The  equations  contained  in  the  text  are  designed  to  be  employed  in  the 
solution  of  the  problems. 

It  should  constantly  be  borne  in  mind  that,  as  far  as  possible,  every 
question  and  principle  should  be  submitted  to  Nature  for  a  direct  answer 
by  means  of  an  experiment.  Pupils  should  be  encouraged  to  try  the 
simple  illustrations  necessary.  The  scholar  who  brings  in  a  bit  of 
apparatus  made  by  himself,  does  better  than  if  he  were  merely  to 
memorize  pages  of  text.  The  objective  or  inductive  method  has  been 
largely  adopted  in  this  book  in  order  to  lead  the  pupil  thus  to  question 
Nature  and  so  verify  each  principle  for  himself.  Where,  however,  it 
seemed  that  a  subject  could  be  more  easily  apprehended  by  using  the 
didactic  method,  the  author  has  not  hesitated  to  adopt  it.  The  true 
teacher  is  not  the  slave  of  any  system,  but  employs  in  each  case  the 


X  SUGGESTIONS   TO   TEACHERS. 

one  that  best  subserves  his  end.  Moreover,  our  pupils  are  not  to 
become  discoverers,  and  so  need  not  necessarily  pursue  the  oftentimes 
tedious  path  of  original  investigation  ;  while  all  the  powers  of  the  mind 
should  be  developed  harmoniously  with  that  of  observation.  Still, 
where  the  didactic  method  of  presentation  is  employed,  the  pupil 
should,  wherever  possible,  perform  the  experiments,  and  so  be  enabled 
to  grasp  the  fact  or  principle  as  he  cannot  by  any  abstract  description, 
however  vivid. 

The  following  works,  to  which  the  author  acknowledges  his  obliga- 
tion for  valuable  material,  will  be  useful  to  teacher  as  well  as  pupil, 
in  furnishing  additional  illustrations  and  in  elucidating  difficult  sub- 
jects, viz. :  Tail's  Recent  Advances  in  Physical  Science  ;  Arnott's  Ele- 
ments of  Physics'  (7th  ed.) ;  Stewart's  Elementary  Physics,  Conserva- 
tion of  Energy,  and  Treatise  on  Heat  ;  Atkinson's  Deschanel's  Natural 
Philosophy ;  Lockyer's  Guillemin's  Forces  of  Nature  ;  Herschel's  In- 
troduction to  the  Study  of  Physical  Science;  Tomlinson's  Introduction 
to  the  Study  of  Natural  Philosophy  ;  Pepper's  Play-book  of  Science  ; 
Beale's  How  to  Work  with  the  Microscope  ;  Schellen's  Spectrum 
Analysis;  Lockyer's  The  Spectroscope  and  Studies  in  Spectrum 
Analysis  ;  Airy's  Geometrical  Optics  ;  Nugent's  Optics;  Chevreul  on 
Colors  ;  Thomson  &  Tail's  Natural  Philosophy  ;  Maxwell's  Electricity 
and  Magnetism  ;  Faraday's  Forces  of  Matter  ;  Youmans's  Correlation 
of  Physical  Forces  ;  Maury's  Physical  Geography  of  the  Sea  ;  Atkin- 
son's Ganot's  Physics  ;  Silliman's  Physics  ;  Tyndall's  Lectures  on 
Light,  Heat,  Sound,  Electricity,  and  Forms  of  Water;  Snell's  Olm- 
sted's  Philosophy  (revised  edition)  ;  Loomis's  Meteorology  ;  Miller's 
Chemical  Physics;  Cooke's  Religion  and  Chemistry,  and  also  nu- 
merous works  named  in  the  Reading  References  at  the  close  of  each 
general  division.  They  may  be  procured  of  the  publishers  of  this 
book.  The  pupil  should  continually  be  impressed  with  the  thought 
that  the  text-book  only  introduces  him  to  a  subject,  which  he  should 
seek  every  opportunity  to  pursue  in  larger  works  and  in  treatises  on 
special  topics. 

As  heretofore,  the  author  will  be  pleased  to  correspond  with  teach- 
ers concerning  the  apparatus  for  the  performance  of  the  experiments, 
or  with  reference  to  any  of  the  "  Practical  Questions." 


THEORIES. 


SINCE  the  revision  of  this  book  in  1878  there  have  been  many 
discoveries  made  in  Physical  Science.  Some  of  these  are  now 
essential  to  the  proper  conception  of  even  elementary  principles,  while 
others  are  interesting  as  opening  up  fresh  fields  of  investigation. 
Experience  has  also  suggested  novel  illustrations,  as  well  as  brought 
old  truths  into  prominence. 

(P.  70.)  "  The  arms  of  a  lever  are  the  two  portions  of  it  intermedi- 
ate, respectively,  between  the  fulcrum  and  the  power,  and  between  the 
fulcrum  and  the  weight.  If  the  lever  is  bent,  or  if,  though  straight,  it 
is  not  at  right  angles  to  the  lines  of  action  of  the  power  and  weight,  it 
is  necessary  to  distinguish  between  the  arms  of  the  lever  as  above 
defined  and  the  arms  of  the  power  and  the  weight  regarded  as  forces 
which  have  moments  round  the  fulcrum.  In  this  latter  case  the  arms 
are  the  perpendicitlars  dropped  from  the  fulcrum  upon  the  lines  of  action 
of  the  power  and  weight." 

(P.  130.)  An  interesting  illustration  of  the  reflection  of  sound  is 
found  at  the  so-called  Echo  River,  of  the  Mammoth  Cave,  Ky.  Sound- 
ing in  succession  the  notes  G,  E,  C,  at  the  middle  of  the  tunnel,  the 
boatman  receives  the  echoes,  all  mingled  into  a  full  chord,  for  eight  or 
ten  seconds  afterward. 

(P.  164.)  Langley's  recent  experiments  near  the  summit  of  Mount 
Whitney  place  the  maximum  of  the  heat  curve  (Fig.  159)  in  the  orange 
or  orange-yellow  instead  of  the  ultra-red.  "  The  sun's  most  intense 
radiations  are  not  the  invisible  ones  as  has  been  so  long  supposed,  but 
the  wave-length  representing  the  maximum  of  heat  does  not  differ 
widely  from  that  representing  the  maximum  of  light."  While  the 
pupil,  for  convenience,  uses  the  terms  heat,  light,  and  chemical  rays, 
he  should  bear  in  mind  the-truth  that  these  rays  differ  not  in  quality, 
but  only  in  pitch. 


Xll  FRESH   FACTS   AND  THEORIES. 

(P.  174.)  The  full  explanation  of  stereoscopic  relief  is  not  so  sim- 
ple as  that  indicated  by  Fig.  172.  The  effect  of  solidity,  or  depth  in 
space,  is  indeed  due  to  the  apparent  blending  of  two  slightly  different 
pictures,  by  causing  the  image  of  one  of  them  to  be  formed  on  one 
retina  and  of  the  other  on  the  corresponding  part  of  the  other  retina. 
But  the  apparent  locality  of  the  combined  external  picture  is  not  deter- 
mined by  the  meeting  of  visual  lines  at  C,  as  was  long  thought  true. 
It  is  merely  an  illusion  of  judgment.  The  pictures  A  and  B  (Fig.  172) 
may  be  so  far  apart  that  the  visual  lines  become  parallel  or  even  diver- 
gent. The  combined  image  then  appears  still  in  front,  but  farther 
away,  larger,  and  deeper.  By  crossing  the  visual  lines  so  that  the 
right  eye  is  directed  to  A  and  the  left  eye  to  B  (Fig.  173),  the  image 
appears  smaller  and  nearer.  The  perspective  is  now  reversed  so  that 
the  tunnel  appears  like  a  mutilated  cone  floating  in  the  air,  with  the 
smallest  part  nearest,  while  two  companion  tunnels  remain,  one  on 
each  side.  (See  art.  on  Stereoscope,  Pop.  Sc.  Monthly,  May  &  June,  '82.) 

(P.  194.)  The  disks  in  Grookes's  Radiometer  are  now  made  of 
aluminium  rather  than  pith,  the  object  being  to  obtain  a  maximum 
absorption  of  heat  on  one  face  which  is  covered  with  lamp-black,  and 
minimum  absorption  on  the  other  which  is  therefore  best  made  of  a 
bright  but  light  metal.  Mica  also  is  used. 

(P.  194.)  In  the  course  of  Prof.  Langley's  experiments  upon  Mount 
Whitney,  water  was  boiled  by  exppsing  it  in  a  copper  vessel  covered 
by  a  pane  of  window-glass,  to  the  direct  rays  of  the  sun.  This  shows 
how  many  of  the  heat-rays  of  the  sunbeam  are  stricken  down  by  the 
air  before  reaching  low  levels,  but  may  be  utilized  at  high  elevations. 
So  that,  paradoxical  as  it  may  seem,  it  is  certain  that,  were  the  atmo- 
sphere removed,  the  earth  would  receive  far  more  heat  and  yet  be  much 
colder  than  now.  (See  American  Journal  of  Science,  March,  1883.) 

(P.  201.)  "  Numerous  observations  made  in  recent  years  show  that 
the  bottom  of  the  ocean,  even  in  equatorial  regions,  is  at  a  temperature 
not  much  higher  than  that  at  which  fresh  water  freezes.  This  cold 
water  has  doubtless  found  its  way  along  the  depths  of  the  sea  from  the 
polar  regions,  while  a  general  flow  from  equator  to  poles  is  taking 
place  nearer  to  the  surface.  In  connection  with  oceanic  circulation  it 
is  to  be  noted  that  sea  water  (unlike  fresh  water),  when  cooled,  con- 
tinues to  contract  until  it  reaches  its  freezing  point." 


I. 

INT<&  O  <D  17  C  TTOJV. 


"  We  have  no  reason  to  believe  that  the  sheep  or  the  dog,  or  indeed  any 
of  the  lower  animals,  feel  an  interest  in  the  laws  by  which  natural  phe- 
nomena are  regulated.  A  herd  may  be  terrified  by  a  thunder-storm  ;  birds 
may  go  to  roost,  and  cattle  return  to  their  stalls  during  a  solar  eclipse ;  but 
neither  birds  nor  cattle,  so  far  as  we  know,  ever  think  of  inquiring  into 
the  causes  of  these  things.  It  is  otherwise  with  man.  The  presence  of 
natural  objects,  the  occurrence  of  natural  events,  the  varied  appearances  of 
the  universe  in  which  he  dwells,  penetrate  beyond  his  organs  of  sense,  and 
appeal  to  an  inner  power  of  which  the  senses  are  the  mere  instruments  and 
excitants.  No  fact  is  to  him  either  final  or  original.  He  cannot  limit 
himself  to  the  contemplation  of  it  alone,  but  endeavors  to  ascertain  its 
position  in  a  series  to  which  the  constitution  of  his  mind  assures  him  it 
must  belong.  He  regards  all  that  he  witnesses  in  the  presznt  as  the  efflux 
anu,  sequence  of  something  that  has  gone  before,  and  as  the  source  of  a 
system  of  events  which  is  to  follow.  The  notion  of  spontaneity,  by  which 
in  his  ruder  state  he  accounted  for  natural  events,  is  abandoned  ;  the  idea 
that  nature  is  an  aggregate  of  independent  parts  also  disappears,  as  the 
connection  and  mutual  dependence  of  physical  powers  become  more  and  more 
manifest ;  until  he  is  finally  led  to  regard  Nature  as  an  organic  whole,  as 
a  body  each  of  whose  members  sympathizes  with  the  rest,  changing,  it  is 
true,  from  age  to  age,  but  without  any  real  break  of  continuity,  or  inter 
ruption  of  the  fixed  relations  of  cause  and  effect." — TYNDALL. 


ANALYSIS  OF  THE  INTRODUCTION, 


GENERAL  DEFINITIONS. 


GENERAL  PROPERTIES 
OF  MATTER. 


lll.SPECIFICPROPERTIES 
OF  MATTER. 


f  1.  Of  Matter,  body  and  substance, 

2.  General  and  Specific  Properties 

3.  The  Atomic  Theory. 

4   Physical  and  Chemical  Changes 

5.  Physical  and  Chemical  Forces. 

6.  Physical  and  Chemical  Proper- 

ties. 

7.  Definition  of  Physics  and  Chem- 

istry. 

1.  Magnitude. 

2.  Impenetrability. 

3.  Divisibility. 

4.  Porosity. 

5.  Inertia. 

I  6.  Indestructibility. 

r  1.  Ductility. 

2.  Malleability. 

3.  Tenacity. 

C  (1.)  Compression. 

(2.)  Expansion. 

4.  Elasticity,  -i  v 

|  (3.)  Torsion. 

[  (4.)  Flexure. 

5.  Hardness. 

6.  Brittleness. 


«.  ^  y 

THE 

UNIVERSITY 

OF 


I.     GENERAL     DEFINITIONS. 

1.  Matter. — Whatever  occupies  space  is  called  matter. 
A  definite  portion  of  matter  is  termed  a  body.     Examples  : 
a  lake,  a  dew-drop,  a  quart  of  oil,  an  anvil,  a  pendulum. 
A  particular  kind  of  matter  is  styled  a  substance.     Exam- 
ples :  gold,  wood,  stone,  oxygen. 

2.  General  and  Specific    Properties.  —  A  general 
property  of  matter  is  a  quality  that  belongs  to  all  substances. 
Example  :    divisibility.       A  specific  property  is  one  which 
distinguishes  particular  substances.     Examples  :  the  yellow 
color  of  gold,  the  brittleness  of  glass,  the  sweetness  of  sugar. 
These  properties  are  so  distinctive  that  we  say,  "yellow  as 
gold,"  "brittle  as  glass,"  "sweet  as  sugar." 

3.  The  Atomic  Theory  supposes 

(1.)  That  the  smallest  particle  of  matter  we  can  see  is 
composed  of  still  smaller  particles  or  molecules  (tiny  masses),* 
each  possessing  the  specific  properties  of  the  substance  to 
which  it  belongs. 

(2.)  That  each  molecule  consists  of  two  or  more  yet 
minuter  portions,  called  atoms,\  which  cannot  be  changed 

*  A  molecule  is  a  group  of  atoms  held  together  by  chemical  force,  and  is  the 
smallest  particle  of  a  substance  which  can  exist  by  itself.  Even  in  a  simple  substance, 
i.  e,,  one  in  which  the  atoms  are  all  of  one  Hnd,  it  is  thought  that  they  are  clustered 
in  molecules.  (See  Chemistry,  p.  20.)  In  water,  the  molecules  are  the  small  masses 
which,  when  driven  apart,  form  steam.  In  a  gas,  they  move  like  so  many  worlds 
through  space,  and  striking  against  the  sides  of  the  containing  vessel,  produce  the 
pressure  of  the  gas  to  escape. 

t  Animalcules  furnish  a  striking  illustration  of  the  minuteness  of  atoms.  In  the 
drop  of  stagnant  water  that  clings  to  the  point  of  a  needle,  swarming  legions  swim 
as  in  an  ocean,  full  of  life,  frisking,  preying  upon  one  another,  waging  war,  and  re- 
enacting  the  scenes  of  the  great  world  about  them.  These  tiny  animals  possess 
organs  of  digestion  and  assimilation.  Their  food,  coursing  in  infinitely  minute  chan- 
nels, must  be  composed  of  solid  as  well  as  liquid  matter ;  and  finally,  at  the  lowest 
extreme  of  this  descending  series,  we  come  to  the  atoms  of  which  the  matter  itself  is 
composed. 


14  INTRODUCTION. 

by  any  material  force.  Examples  :  a  molecule  of  water  is 
made  up  of  two  atoms  of  hydrogen  and  one  of  oxygen.  A 
molecule  of  salt  consists  of  one  atom  of  chlorine  and  one  of 
sodium.  The  smallest  piece  of  salt  contains  many  molecules. 
By  dissolving  in  water,  we  divide  it  into  its  separate  mole- 
cules, and  the  solution  has  a  briny  taste,  because  each  one 
possesses  the  savor  of  salt. 

4.  Physical  and  Chemical  Changes. — A  physical 
change  is  one  that  does  not  destroy  the  molecule,  and  so 
does  not  alter  the  specific  properties  of  a  substance.     Exam- 
ples :  the  falling  of  a  stone  to  the  ground,  the  dissolving  of 
sugar  in  water.     A  chemical  change  is  one  that  makes  new 
molecules  and  so  destroys  the  specific  properties  of  a  sub- 
stance.    Examples  :  the  rusting  of  iron,  the  burning  of  coal. 

5.  Physical  and  Chemical  Forces. — A.  physical,  force 
is  one  that  produces  a  physical  change  in  matter.     Exam- 
ples :   heat  when  it  turns  water  into  steam,  light  when  it 
illumines  a  room,  magnetism  when  a  knife-blade  attracts  a 
needle.     A  chemical  force  is  one  that  produces  a  chemical 
change.     Example  :  affinity  when  it  converts  sand  and  soda 
into  glass. 

One  kind  of  force  sometimes  develops  another.  Exam- 
ples :  heat  turns  sugar  into  charcoal  and  steam,  light  causes 
chemical  changes  in  vegetation,  chemical  force  corrodes 
zinc  and  thus  sets  free  electricity. 

6.  Physical  and  Chemical  Properties. — A  physical 
property  is  one  that  can  exist  in  a  substance  without  essen- 
tially changing  the  molecular  structure  of  that  or  of  any  other 
substance.      Examples :   melting  point,   color,  weight.     A 
chemical  property  is  one  that  determines  the  character  of  the 
chemical  change  of  which  a  substance  is  susceptible,  or  the 
chemical  effect  it  may  exert  upon  other  substances.     Exam- 
ples :  the  power  of  gunpowder  to  explode,  the  tendency  of 
wood  to  unite  with  the  oxygen  of  the  air  and  so  decay, 
the  reciprocal  action  of  soda  and  cream  of  tartar  to  cause 
effervescence. 


GENERAL    PROPERTIES    OF    MATTER.  15 

7.  Physics  and  Chemistry. — The  former  treats  of 
phenomena  in  which  there  is  a  physical  change  in  matter ; 
the  latter,  of  those  in  which  there  is  a  chemical  change. 
The  unit  of  the  physicist  is  the  molecule  ;  of  the  chemist, 
the  atom.  As  both  kinds  of  force  and  properties  reside  in 
every  substance,  and  every  substance  is  susceptible  of  both 
kinds  of  change,  the  two  subjects  are  intimately  connected. 

PRACTICAL  QUESTIONS.— Name  some  specific  property  of  coal:  ink-  chalk; 
grass ;  tobacco ;  snow.  My  knife-blade  is  magnetized,  so  that  it  will  pick  up  a  needle ; 
is  that  a  physical  or  chemical  change  ?  Is  it  treated  in  Physics  or  Chemistry  ?  Is  the 
burning  of  coal  a  physical  or  chemical  change  ?  The  production  of  steam  ?  The  for- 
mation of  dew  ?  The  falling  of  a  stone  ?  The  growth  of  a  tree  ?  The  flying  of  a  kite  ? 
The  chopping  of  wood  ?  The  explosion  ol  powder  ?  The  boiling  of  water  ?  The 
melting  of  iron  ?  The  drying  of  clothes  ?  The  freezing  of  water  ?  The  dissolving  of 
sugar?  The  forging  a  nail?  The  making  of  bread  ?  The  sprouting  of  a  seed  ?  The 
decay  of  vegetables  ?  The  condensation  of  steam  ? 


II.     GENERAL     PROPERTIES     OF 
MATTER. 

The  principal  general  properties  of  matter  are  extension, 
impenetrability,  divisibility,  porosity,  inertia,  and  inde- 
structibility. *  We  cannot  imagine  a  body  which  does  not 
possess  them  all. 

1.  Extension  is  the  property  of  occupying  space  or 
having  volume.  Size  is  the  quantity  of  space  a  body  fills. 
A  body  has  three  dimensions — length,  breadth,  and  thick- 
ness. To  measure  these,  some  standard  is  required.  England 
and  the  United  States  have  chosen  an  arbitrary  one  called 
the  yard.  France  has  adopted  the  metre,  which  is  about 
of  an  entire  meridian  of  the  earth.  This  is  a 


4  0,0  0  0.0  0  (I 


The  tirbt  two  of  these,  serving  to  defiue  matter,  are  ita  essential  attrih^  tea. 


16  INTRODUCTION. 

unit  on  which  is  based  a  decimal  system  that,  because  of  its 
simplicity,  is  steadily  growing  in  favor. 

2.  Impenetrability  is  the  property  of  so  occupying 
space  as  to  exclude  all  other  matter.*    No  two  bodies  can 
occupy  the  same  space  at  the  same  time.     A  book  lies  upon 
the  table  before  me ;   no  human  power  is  able  to  place 
another  in  the  same  spot,  until  the  first  book  is  removed. 
I  attempt  to  fill  a  bottle  through  a  closely-fitting  funnel ; 
but  before  the  liquid  can  run  in,  the  air  must  gurgle  out, 
or  the  water  will  trickle  down  the  outside  of  the  bottle. 

3.  Divisibility  is  that  property  which  allows  a  body  to 
be  separated  into  parts.     The  extent  to  which  the  divisibility 
of  matter  may  be  carried  is  almost  incredible,  f     Example  : 
a  gram  of  strychnine  will  flavor  1,750,000  grains  of  water  ; 
hence  there  will  be  in  each  grain  of  the  liquid  only  i  ^  g  }>0  o¥ 
of  a  grain  of  strychnine,  yet  this  amount  can  be  distinctly 
tasted. 

4.  Porosity  is  the  property  of  having  pores.     By  this  is 
meant  not  indeed  the  sensible  pores  to  which  we  refer  when 
in  common  language  we  speak  of  a  porous  body,  as  bread, 
wood,   unglazed    pottery,   a   sponge,   etc.,  but   the  finer  or 
physical  pores.     The  latter  are  as  invisible  to  the  eye  as  the 

*  IE  common  language,  we  say  a  needle  penetrates  cloth,  a  nail  enters  wood,  etc. : 
but  a  moment's  examination  shows  that  they  merely  push  aside  the  fibres  of  the  cloth 
or  wood,  and  so  press  them  closer  together.  With  care  we  can  drop  a  quarter  of  a 
pound  of  shingle-nails  into  a  tumbler  brimfull  of  water,  without  causing  it  to  over- 
flow. The  surface  of  the  water,  however,  becomes  convex. 

t  Newton  estimated  that  the  film  of  a  soap-bubble  at  the  instant  of  breaking  is 
less  than  5^000  of  an  inch  thick.  Pure  water  will  acquire  the  requisite  viscidity  for 
making  bubbles  by  adding  only  T£o  part  of  soap.  It  is  evident  that  there  must  be  at 
least  one  molecule  of  soap  in  every  cubic  aa^poo  of  an  inch  of  the  film,  and  that  the 
molecule  must  be  smaller  than  one-hundredth  of  a  cubic  ^fosoos  of  an  inch.  *•  «-i  than 
rTS;i__  trillionths  of  a  cubic  inch.  Now  a  molecule  of  soft-soap  (if  it  is  a  pure  potas- 
sium stearate,  Chemistry,  p.  219)  contains  56  atoms,  and  this  point  must  be  reached 
before  we  come  to  the  possible  limit  of  divisibility. — Some  idea  of  the  vastness 
expressed  by  the  word  trillion  may  be  derived  from  the  estimate  that  if  Adam,  at  his 
creation,  had  commenced  to  count  one  every  second  of  time,  he  would  not  yet  have 
completed  the  first  quarter  of  a  trillion  ;  and  if  Eve  had  come  to  his  relief,  and  they 
had  counted  day  and  night,  they  would  not  see  the  end  of  their  task  for  1U,000  years 
to  come.  (See  also  note  on  electric  sparks,  p.  236.) 


GENERAL    PROPERTIES    OF    MATTER.  17 

atoms  themselves,  and  are  caused  by  the  fact  that  the  mole- 
cules of  which  a  body  is  composed  are  not  in  actual  contact, 
but  are  separated  by  minute  spaces.*  Ex.  :  to  a  bowl-full 
of  water  it  is  easy  to  add  a  quantity  of  fine  salt  without  the 
liquid  running  over.  Only  care  must  be  taken  to  drop  in 
the  salt  slowly,  giving  time  for  it  to  dissolve  and  the  bubbles 
of  air  to  pass  off.  When  the  water  has  taken  up  all  the  salt 
it  will,  we  can  still  add  other  soluble  solids,  f— In  test- 
ing large  cannon  by  hydrostatic  pressure  (p.  85),  water  is 
forced  into  the  gun  until  it  oozes 
through  the  thick  metal  and  cov- 
ers the  outside  of  the  gun  like 
froth,  then  gathers  in  drops  and 
runs  to  the  ground  in  streams.  J 

The  process  of  filtering,  so  much 
employed  by  druggists,  depends 
upon  this  property;  the  liquid 
slowly  passes  through  the  pores 
of  the  filter,  leaving  the  solid 
portions  behind. — Water,  in  Na- 
ture, is  thus  purified  by  perco- 
lating through  beds  of  sand  and 


*  These  spaces  are  so  small  that  they  cannot  be  discerned  with  the  most  power- 
ful microscope,  yet  it  is  thought  that  they  are  very  large  when  compared  with  the 
size  of  the  atoms  themselves.  If  we  imagine  a  being  small  enough  to  live  on  one  of 
the  atoms  near  the  centre  of  a  stone,  as  we  live  on  the  earth,  then  we  are  to  suppose 
that  he  would  see  the  nearest  atoms  at  great  distances  from  him,  as  we  see  the  moon 
and  stars,  and  might  perchance  have  need  of  a  fairy  telescope  to  examine  them,  as 
we  investigate  the  heavenly  bodies. 

t  In  this  case  we  suppose  that  the  particles  of  salt  are  smaller  than  those  of  water, 
and  those  of  the  different  substances  used  are  smaller  than  those  of  salt.  The  parti- 
cles of  salt  fill  the  spaces  between  the  particles  of  water,  and  the  others  occupy  the 
still  smaller  spaces  left  between  the  particles  of  salt.  We  may  better  understand 
this  if  we  suppose  a  bowl  filled  with  oranges.  It  will  hold  a  quantity  of  peas,  then  of 
gravel,  then  of  fine  sand,  and  lastly  some  water. 

%  In  the  course  of  some  experiments  performed  during  the  18th  centuiy  at  the 
Florence  Academy,  Italy,  hollow  globes  of  silver  were  filled  with  water  and  placed  in 
a  screw-press.  The  spheres  being  flattened,  their  size  was  diminished,  and  the  water 
oozed  through  the  pores  of  the  metal.  The  philosophers  of  the  day  thought  this  to 
show  that  water  is  incompressible.  We  now  see  that  it  proved  only  that  silver  baa 
pores  larger  than  the  molecules  of  water. 


18  .      INTRODUCTION. 

gravel. — Cisterns  for  filtering  water  have  a  brick  partition 
in  the  middle.  The  water  is  cleansed  as  it  soaks  through 
the  porous  brick.  Small  filters  are  frequently  made  of  a 
cask  nearly  filled  with  gravel  and  charcoal ;  the  water  is 
poured  in  a  little  reservoir  at  the  top  and  drawn  off  at  the 
bottom  by  a  faucet. 

5.  Inertia  is  the  negative  property  of  passiveness.  *    Mat- 
ter has  no  power  of  putting  itself  in  motion  when  at  rest, 
nor  of  coming  to  rest  when  in  motion.     A  body  will  never 
change  its  place  unless  moved,  and  if  once   started  will 
move  forever  unless  stopped.     Ex.  :  If  we  leave  the  room, 
and  on  our  return  find  a  book  missing,  we  know  some  one 
has  taken  it — the  book  could  not  have  gone  off  of  itself. 

6.  Indestructibility  is  the  property  which  renders  mat- 
ter incapable  of  being  destroyed.     No  particle  of  matter  can 
be  annihilated,  except  by  God,  its  creator.     We  may  change 
its  form,  but  we  cannot  deprive  it  of  existence.     Ex.  :  We 
cut  down  a  tree,  saw  it  into  boards,  and  build  a  house.    The 
house  burns,  and  only  little  heaps  of  ashes  remain.     Yet  in 
the  ashes,  and  in  the  smoke  of  the  burning  building,  exist 
the  identical  atoms,  which  have  passed  through  these  various 
forms  unchanged,  f 

*  The  common  idea  of  inertia  is  that  matter  actively  resists  any  change ;  and  that 
when  we  lift  a  heavy  stone,  for  example,  we  must  overcome  the  determined  opposi- 
tion of  the  body  to  be  moved.  Matter  possesses  no  such  property.  The  seeming 
obstinacy  is  due  to  the  fact  that  time  is  required  to  impart  motion  to  a  body  at  rest, 
and  to  overcome  the  momentum  of  a  body  in  motion.  The  illustrations  ordinarily 
given  of  inertia  are  really  examples  of  a  law  of  motion.  We  are  also  accustomed 
to  think  a  body  is  more  inclined  to  rest  than  to  motion  ;  and  so,  while  we  see  how  a 
stone  could  not  throw  itself,  we  find  it  difficult  to  understand  how,  once  thrown,  it 
does  not  stop  itself.  We  shall  see  hereafter  that  several  forces  destroy  its  motion 
and  bring  it  to  rest.  (See  pp.  28,  29,  and  Questions  55-62,  p.  89.) 

t  Walter  Raleigh,  while  smoking  in  the  presence  of  Queen  Elizabeth,  offered  to 
bet  her  majesty  that  he  could  tell  the  weight  of  the  smoke  that  curled  upward  from 
his  pipe.  The  wager  was  accepted.  Raleigh  quietly  finished,  and  then  weighing  the 
ashes,  subtracted  this  amount  from  the  weight  of  the  tobacco  he  had  placed  in  the 
pipe,  thus  finding  the  weight  of  the  smoke.  When  we  reach  the  subject  of  combus- 
tion in  chemistry,  we  shall  be  able  to  detect  Raleigh's  mistake.  The  smoke  and  the 
ashes  really  weighed  more  than  the  original  tobacco,  since  the  oxygen  of  the  air  had 
combined  with  the  tobacco  in  burning. 


SPECIFIC    PROPERTIES    OF    MATTER.  19 


III.     SPECIFIC     PROPERTIES     OF 
MATTER. 

Among  the  most  important  specific  properties  of  matter 
are  ductility,  malleability,  tenacity,  elasticity,  hardness,  and 
brittleness. 

1.  Ductility. — A  ductile  body  is  one  which  can  be  drawn 
into  wire.     Fig.  3  represents  a  machine  for  making  wire. 
B  is  a  steel  draw- 
ing-plate   pierced  Fl°- 3- 
with  a    series    of 
gradually    dimin- 
ishing   holes.      A 
rod  of  iron,  A,  is 
hammered  at  the 
end  so  as  to  pass 
through  the  larg- 
est.     It    is    then 

grasped  by  a  pair  of  pincers,  0,  and,  by  turning  the  crank 
D,  is  drawn  through  the  plate,  diminished  in  diameter  and 
proportionately  increased  in  length.  The  tenacity  of  the 
metal  is  greatly  improved  by  the  process  of  drawing,  so  that 
a  cable  of  fine  wire  is  stronger  than  a  chain  or  bar  of  the 
same  weight.  Gold,  silver,  and  platinum  are  the  most  duc- 
tile metals.  A  silver  rod  an  inch  thick,  covered  with  gold- 
leaf,  may  be  drawn  to  the  fineness  of  a  hair  and  yet  retain  a 
perfect  coating  of  gold,  3  oz.  of  the  latter  metal  making  100 
miles  of  the  gilt-thread  used  in  embroidery.  Platinum  wire 
has  been  drawn  so  fine  that,  though  it  is  nearly  three  times 
as  heavy  as  iron,  a  mile's  length  weighed  only  a  single  grain. 
(Chemistry,  p.  170.) 

2.  Malleability. — A  malleable  body  is  one  which  can  be 
hammered  or  rolled  into  sheets.  Ex.  :  Gold  may  be  beaten 
until  it  is  only  ?60i000  of  an  inch  thick.  It  would  require 


INTRODUCTION. 


PIG.  4. 


1800  such  leaves  to  equal  the  thickness  of  common  printing' 
paper.*  Copper  is  so  malleable,  that  a  workman  can  ham- 
mer out  a  kettle  from  a  solid  block. 

3.  Tenacity. — A  tenacious  body  is  one  which  cannot 
easily  be  pulled  apart.     Iron  possesses  this   quality  in  a 
remarkable  degree.     Steel  wire  will  sustain  the  weight  of 
about  7-J-  miles  of  itself . 

4.  Elasticity  is  of  four  kinds,  according  as  a  body  tends 
to   resume   its   original   form   when    compressed,    extended, 
twisted,  or  lent. 

(I.)  ELASTICITY  OF  COMPRESSION. — Many  solids,  as  iron, 
glass,  and  caoutchouc,  are  highly  elastic.  Ex.  :  Spread  'a 

thin  coat  of  oil  on  a  smooth  mar- 
ble slab.  If  an  ivory  ball  be 
dropped  upon  it,  the  size  of  the 
impression  will  vary  with  the 
distance  at  which  the  ball  is  heJd 
above  the  table.  This  shows  that 
the  ivory  is  flattened,  somewhat 
like  a  soap-bubble  when  it  strikes 
a  smooth  surface  and  rebounds. 

Liquids  are  condensed  with 
great  difficulty,  so  that  for  a  long 
time  they  were  considered  in- 
compressible. When  the  force  is 
removed,  they  regain  their  exact 
volume,  and  are  therefore  perfectly  elastic. 

Gases  are  easily  compressed,  and  are  also  perfectly  elastic. 
A  pressure  of  15  Ibs.  to  the  square  inch  reduces  the  volume  of 

*  An  Ingot  of  gold  is  passed  many  times  between  steel  rollers,  which  are  so  ad- 
justed as  to  be  constantly  brought  nearer  together.  The  metal  is  thus  reduced  to  a 
ribbon  about  g^  of  an  inch  thick.  This  is  cut  into  inch  squares,  150  of  which  are 
piled  up  alternately  with  leaves  of  stronsrpaper  four  inches  square.  A  workman  with 
a  16-lb.  hammer  beats  the  pile  until  the  gold  is  spread  to  the  size  of  the  leaves.  Each 
piece  is  next  quartered,  and  the  600  squares  are  placed  between  leaves  of  goldbeaters' 
skin  and  pounded.  They  are  then  taken  out,  spread  by  the  breath,  cut.  and  the  2  400 
squares  pounded  as  before.  They  are  finally  trimmed  and  placed  between  tissue- 
paper  in  little  books,  each  of  which  contains  25  gold  leaves. 


SPECIFIC     PROPERTIES    OF    MATTER. 


Fro.  5. 


water  only  20}00,  whereas  it  diminishes  the  volnme  of  a 
gas  £.  A  gas  may  be  kept  compressed  for  years,  but  on 
being  released  will  instantly  return  to  its  original  form. 

(2.)  ELASTICITY  OF  EXPANSION  is  possessed  largely  by 
solids,  slightly  by  liquids,  and  not  at  all  by  gases.  Ex..: 
India-rubber,  when  stretched,  tends  to  fly  back  to  its  origi- 
nal dimensions.  A  drop  of  water  hanging  to  the  nozzle  of 
a  bottle  may  be  touched  by  a  piece  of  glass  and  drawn  out 
to  considerable  length,  but  when  let  go  it  will  resume  its 
spherical  form.  Gases  when  extended  manifest  no  tendency 
to  return  to  their  former  shape. 

(3.)  ELASTICITY  OF  TORSION  is  the  tendency  of  a  thread 
or  wire  which  has  been  twisted,  to  un- 
twist again.     It  is  a  delicate  test  of  the 
strength  of  a  force  (Fig.  5). 

(4.)  ELASTICITY  OF  FLEXURE  is  the 
property  ordinarily  meant  by  the  term 
elastic.  Many  solids  possess  this  quality, 
within  certain  limits,  to  a  high  degree. 
Swords  have  been  made  which  could  be 
bent  into  a  circle  without  breaking. 
Watch-springs,  bows,  cushions,  etc.,  are 
useful  because  of  their  elasticity. 

,5.  Hardness. — One  body  is  harder 
than  another  when  it  will  scratch  or  in- 
dent it.     This  property  does  not  depend 
on  density.*    Ex.  :  Gold  is  about  2J  times  denser  than  iron, 
yet  it  is  much  softer. — Mercury  is  a  liquid,  yet  it  is  almost 
twice  as  dense  as  steel. — The  diamond  is  the  hardest-known 
substance,  yet  it  is  not  one-third  as  heayy  as  lead. 

6.  Brittleness. — A  brittle  body  is  one  that  is  easily 
broken.  This  property  is  a  frequent  characteristic  of  hard 
bodies.  Ex.  :  Glass  will  scratch  pure  iron,  yet  it  is  extremely 
brittle. 


*  A  dense  body  has  its  molecules  closely  compacted.  The  word  rare,  the  opposite 
of  dense,  is  applied  to  gases.  Mass,  or  the  quantity  of  matter  a  body  contains, 
should  be  distinguished  from  weight  or  size  (notes,  pp.  53,  2&7). 


22  INTRODUCTION. 


SUM  MARY. 

Matter  is  that  which  occupies  space.  A  separate  portion  is  called  a 
body,  and  a  particular  kind  a  substance.  A  general  property  of  matter 
belongs  to  all  substances,  and  a  specific  one  to  particular  kinds.  Mat- 
ter is  composed  of  very  minute  atoms.  A  group  of  atoms  forms  a 
molecule,  in  which  reside  the  specific  properties  of  a  substance.  A 
physical  change  never  affects  the  molecule,  but  a  chemical  change 
breaks  it  up,  and  so  forms  a  new  substance.  Philosophy  deals  with 
physical  forces  and  changes ;  Chemistry  with  chemical  force  or  attrac- 
tion, and  chemical  changes.  Extension,  impenetrability,  divisibility, 
porosity,  inertia,  and  indestructibility  are  the  principal  general  prop- 
erties of  matter.  Of  these,  extension  is  the  property  of  occupying 
space ;  the  amount  of  space  a  body  fills  is  its  size.  Impenetrability 
prevents  two  bodies  from  occupying  the  same  space  in  the  same  time. 
Divisibility  permits  a  body,  so  far  as  we  know,  to  be  divided  infinitely. 
Porosity  is  caused  by  the  particular  structure  of  a  body  ;  pores  are 
inherent  in  the  constitution  of  a  body  which  consists  of  molecules 
that  do  not  touch.  Inertia  is  that  property  of  matter  which  forbids 
its  changing  its  state  from  motion  to  rest,  or  vice  versa.  It  is  like 
laziness  in  a  human  being.  Indestructibility  prevents  the  extinction 
of  matter  by  man.  Ductility,  malleability,  tenacity,  elasticity,  hard- 
ness, and  brittleness  are  the  principal  specific  properties  of  matter. 
A  ductile  body  can  be  drawn  into  wire  ;  gold,  silver,  and  platinum 
are  the  most  noted  for  this  property.  A  malleable  body  can  be  ham- 
mered into  sheets  ;  gold  possesses  this  quality  in  a  remarkable  degree. 
A  tenacious  body  resists  pulling  apart ;  iron  is  the  best  example. 
An  elastic  body  permits  a  play  of  its  particles,  so  that  they  return  to 
their  original  position  when  the  disturbing  force  is  removed.  A  hard 
body  cannot  easily  be  indented.  A  brittle  body  is  readily  broken. 


HISTORICAL    SKETCH. 

In  ancient  times,  any  seeker  after  truth  was  termed  a  philosopher 
(a  lover  of  wisdom),  and  philosophy  included  all  investigations  concern- 
ing both  mind  and  matter.  In  the  fourth  century  B.  C.,  Plato  assumed 
that  there  are  two  principles,  matter  and  form,  which  by  combining 
produce  the  five  elements,  earth,  air,  fire,  water,  and  ether.  Aristotle, 
his  pupil,  established  the  first  philosophical  ideas  concerning  mattej 


HISTORICAL    SKETCH.  23 

and  space.  But  the  method  of  study  generally  pursued  for  2000  years 
was  one  of  pure  metaphysical  speculation.  Observation  had  no  place, 
but  the  philosophers  made  up  a  theory,  and  then  accommodated  facts 
to  it.  They  guessed  about  the  real  essence  of  things,  as  to  whether 
matter  exists  except  when  perceived  by  the  mind,*  and  how  a  change 
in  matter  can  produce  a  change  in  mind.  In  1620,  Bacon  published  his 
"  Novum  Organum,"  advocating  the  inductive  method  of  studying 
nature.  He  argued  that  the  philosopher  should  seek  to  benefit  man* 
kind,  and  that,  instead  of  wasting  his  time  in  sterile  and  ingenious 
theories  about  the  world  and  matter,  he  should  watch  the  phenomena 
of  life,  gather  facts,  and  then  reasoning  from  effects  back  to  their 
causes,  reach  the  general  law.  This  work  is  commonly  said  to  have 
established  the  modern  method  of  investigation.  Ptolemy,  Archimedes, 
Galileo,  and  other  physicists,  however,  had  long  before  proved  its 
value. 

The  Atomic  Theory  was  propounded  by  Democritus,  in  the  fifth 
century  B.  C.,  and  twenty-two  centuries  later  elaborated  by  Dal  ton,  an 
English  physicist.  The  grander  generalization  and  development  of 
this  law  was  advanced  in  1811  by  Avogadro,  an  Italian,  and  afterward 
extended  by  the  French  philosopher,  Ampere.  The  latter  asserted  that 
"«qual  volumes  of  all  substances,  when  in  the  gaseous  form  and  under 
like  conditions,  contain  the  same  number  of  molecules."  For  half  a 
century  this  view  lay  dormant.  Of  late  it  has  borne  fruit,  and  the 
molecular  theory  has  become  to  Chemistry  what  the  law  of  gravitation 
is  to  Astronomy.  The  labors  of  Thomson,  Cooke,  Tait  and  others  are 
now  building  up  the  whole  superstructure  of  Chemistry  and  Physics 
upon  this  basis. 

The  history  of  the  establishment  of  a  standard  of  measures  is  a 
curious  one.  Anciently,  length  was  referred  to  some  portion  of  the 
human  body,  as  the  foot ;  the  cubit  (the  length  of  the  forearm  from  the 
elbow  to  the  end  of  the  middle  finger) ;  the  finger's  length  or  breadth ; 
the  hand's  breadth  ;  the  span,  etc.  In  England,  Henry  I.  (1120)  ordered 
that  the  ell,  the  ancient  yard,  should  be  the  exact  length  of  his  arm. 
Afterward  a  standard  yard-stick  was  kept  at  the  Exchequer  in  London  ; 
but  it  was  so  inaccurate,  that  a  commissioner,  who  examined  it  in  1742, 
wrote  :  "  A  kitchen  poker  filed  at  both  ends  would  make  as  good  a 
standard.  It  has  been  broken,  and  then  repaired  so  clumsily  that  the 
joint  is  nearly  as  loose  as  a  pair  of  tongs."  In  1760,  Mr.  Bird  carefully 
prepared  a  copy  of  this  for  the  use  of  the  Government.  It  was  not 
legally  adopted  until  1824,  when  it  was  ordered  that  if  destroyed,  it 


*  Dr.  Johnson  once  remarked  to  a  geutleman  who  had  oeen  defending  the  theory 
that  there  is  no  external  world,  as  he  was  going  away,  ''Pray,  sir,  don't  leave  us, 
for  we  may  perhaps  forget  to  think  of  you,  and  then  you  will  cease  to  e?ist." 


24  HISTORICAL    SKETCH. 

should  be  restored  by  a  comparison  with  the  length  of  a  pendulum 
vibrating  seconds  at  the  latitude  of  London.  (Third  law,  p.  60.)  At 
the  great  fire  in  London,  1834,  the  Parliament  House  was  burned,  and 
with  it  Bird's  yard-stick.  Repeated  attempts  were  then  made  to  find 
the  length  of  the  lost  standard  by  means  of  the  pendulum.  This  was 
found  impracticable,  on  account  of  errors  in  the  original  directions.  At 
last  the  British  government  adopted  a  standard  prepared  from  the  most 
reliable  copies  of  Bird's  yard-stick.  A  copy  of  this  was  taken  by 
Troughton,  a  celebrated  instrument-maker  of  London,  for  the  use  of 
our  Coast  Survey.* 

The  French  had  previously  adopted  for  the  length  of  the  legal  foot 
that  of  the  royal  foot  of  Louis  XIV.,  as  perishable  a  standard  as  Henry's 
arm.  When  they  had  established  the  metric  system,  they  found  that 
a  mistake  had  been  made  in  measuring  the  meridian.  The  English 
scientists  discovered  a  difficulty  in  the  calculation  from  the  pendu- 
lum. So  that  both  these  attempts  to  fix  upon  an  absolute  unit  in 
Nature  have  failed,  and  the  French  and  English  systems  are  alike 
founded  upon  arbitrary  standards. 

Consult  Cooke's  "  New  Chemistry,"  chapter  on  Molecules,  etc.; 
Powell's  "History  of  Natural  Philosophy";  Buckley's  "History  of 
Natural  Science";  Whewell's  "History  of  the  Inductive  Sciences"; 
Roscoe's  "John  Dalton  and  his  Atomic  Theory,"  in  Manchester 
Science  Lectures,  '73-4;  "Appleton's  Cyclopaedia,"  Art.  Molecules; 
Outerbridge's  "Divisibility  of  Gold  and  Other  Metals,"  in  Popular 
Science  Monthly,  Vol.  XI,  p.  74 ;  Crookes'  "  The  Radiometer — a  fresh 
evidence  of  a  Molecular  Universe,"  Popular  Science  Monthly,  Vol. 
XIII,  p.  1 ;  Tait's  "Recent  Advances  in  Physical  Science,"  Chap.  XII, 
The  Structure  of  Matter  ;  Hoef er,  "  Histoire  de  la  Physique  et  de  la 
Chimie";  Draper's  "History  of  Intellectual  Development." 

*  This  yard  is  about  ^s  of  an  inch  longer  than  the  British  standard.  According 
to  Act  of  Congress,  sets  of  weights  and  measures  have  been  distributed  to  the  gov- 
ernors of  the  several  States.  The  yards  so  furnished  are  equal  to  that  of  tlie 
Troughton  scale.  We  have  no  national  standard  established  by  law. 


II. 

MOTION    3JV£>  FORCE. 


Rest  is  nowhere.  The  winds  that  come  and  go,  the  ocean  that  uneasily 
throbs  along  the  shore,  the  earth  that  flies  about  the  sun,  the  light  that  darts 
through  space — all  tell  of  a  universal  law  of  Nature.  The  solidest  body 
hides  within  it  inconceivable  velocities.  Even  the  molecules  of  granite  and 
iron  have  their  orbits  as  do  the  stars,  and  revolve  as  ceaselessly. 

No  energy  is  ever  lost.  It  changes  its  form,  but  the  eye  of  philosophy 
detects  it  and  enables  us  to  drive  it  from  its  hiding-place  undiminished.  It 
assumes  Protean  guises,  but  is  everywhere  a  unit.  It  may  disappear  from 
the  earth;  still— 

"  Somewhere  yet  that  atom's  force 
Moves  the  light-poised  universe? 


ANALYSIS. 


MOTION  AND 
FORCE. 


r    1.  DEFINITIONS. 

2.  RESISTANCES  TO  (  (*•) 

MOTION.          j  (2-)  Resistance  of  air 
(  and  water. 

3.  MOMENTUM. 

4.  COMMUNICATION  OF  MOTION. 

5.  THREE  LAWS  OF  MOTION. 

6.  COMPOUND  MOTION. 

7.  COMPOSITION  OF  FORCES. 

8.  RESOLUTION  OF  FORCES. 

9.  MOTION  IN  A  CURVE. 

10.  CIRCULAR  MOTION. 

11.  THE  GYROSCOPE. 

12.  REFLECTED  MOTION. 
18.  ENERGY. 

14.  POTENTIAL  AND  DYNAMIC  ENERGY, 

15.  CONSERVATION  OF  ENERGY. 


MOTION     AND     FORCE. 

1.  Motion  is  a  change  of  place.     All  motion,  as  well  as 
rest,  with  which  we  are  acquainted,  is  relative.     Ex.  :  When 
we  ride  in  the  cars,  we  judge  of  our  motion  by  the  objects 
around  us. — A  man  on  a  steamer  may  be  in  motion  with 
regard  to  the  shore,  but  at  rest  with  reference  to  the  objects 
on  the  deck  of  the  vessel.     Force  is  that  which  produces  or 
tends  to  produce  or  to  destroy  motion.      Velocity  is  the  rate 
at  which  a  body  moves. 

2.  The  Resistances  to  Motion  are  friction  and  the 
resistance  of  air  and  water.     (1.)  Friction  is  the  resistance 
caused  by  the  surface  over  which  a  body  moves.     It  is  of 
great  value  in  common  life.     "Without  it,  nails,  screws,  and 
strings  would  be  useless  ;  engines  could  not  draw  the  cars  ; 
we  could  hold  nothing  in  our  hands  ;  and  we  should  every- 
where walk  as  on  glassy  ice.     (2.)  The  resistance  which  a 
body  meets  in  passing  through  air  or  water  is  caused  by  the 
particles  displaced.  r  It  increases  according  to  the  square  of 
the  velocity.*    Thus,  if  in  running  we jlouble  our  speed,  we 
displace  twice  as  much  air  in  the  same  time,  and  give  to 
each  particle  twice  the  velocity ;  hence  the  resistance  will 
be  quadrupled. 

3.  Momentum,  is  the  name  given  to  the  product  of  the 
mass  of  a  body  multiplied  by  its  velocity  per  second,  ex- 
pressed in  feet.     Ex. :  A  stone  weighing  5  Ibs.,  thrown  with 
a  velocity  of  20  feet  per  second,  has  100  units  of  momen- 
tum.! 

*  This  is  true  at  a  moderate  velocity,  but  at  a  high  speed  some  of  the  medium  is 
carried  with  the  body,  and  the  resistance  increases  faster  than  according  to  v3. 

t  Physicists  make  momentum  =  mass  x  velocity.  The  mass  of  bodies  (note, 
p.  21)  is  proportional  to  weight,  at  the  same  place  on  the  earth's  surface ;  but, 
while  the  mass  remains  the  same  at  different  places,  the  weight  varies  (p.  53).— A 
heavy  body  may  move  slowly  and  yet  have  an  immense  momentum.  Ex.:  An  ice- 
berg, with  a  scarcely  perceptible  motion,  will  crush  a  man-of-war  as  if  it  were  an 


MOTION    AND    FOBCB. 

4.  The  Communication  of  Motion  is  not  instanta- 
neous.*   If  I  press  with  all  my  might  against  a  rock  weigh- 
ing a  ton,  I  fail  to  move  it,  press  I  ever  so  long.     The  force 
is  not  sufficient  to  overcome  the  friction  between  the  rock 
and  the  ground.     If,  however,  we  could  conceive  the  rock 
poised  in  empty  space,  the  least  touch  would  at  once  move 

it  with  a  velocity  proportional  to  Pressuret     if  j  strike  one 

mass 

end  of  a  rail  a  mile  long,  the  tremor  will  take  a  definite 
time  to  reach  the  other  end.  If,  on  the  other  hand,  a 
powerful  engine  suddenly  pulls  at  one  end  of  the  rail,  so  as 
to  draw  it  over  a  consi  ierable  distance  in  a  second,  I  can 
imagine  that  the  other  end  will  move  after  an  almost 
infinitely  short  time ;  but  if  the  engine  drag  the  rail  con- 
tinuously, both  ends  will  have  the  same  velocity,  and  the 
whole  rail  will  move  together. 

5.  Three  Laws  of  Motion. — FIRST.    A  body  set  in  mo- 
tion will  move  forever  in  a  straight  hne,  unless  acted  on  ly 
some  external  force.     This  is  only  another  statement  of  the 
passiveness  of  matter,  or  the  property  of  inertia.     Obvious- 
ly, no  experiment  will  directly  prove  the  law.     There  is  a 
curious  illustration,  however,  in  the  swinging  of  a  pendulum 
under  the  receiver  of  an  air-pump.     The  more  perfectly  the 
air  is  exhausted,  the  longer  it  will  vibrate.     In  the  best 
vacuum  we  can  produce,  it  will  swing  for  twenty-four  hours. 
It  is  supposed  that  if  all  "resistances  to  motion"  vere 
removed,  the  pendulum  would  never  stop. 

egg-shell.— Vessels  lying  at  a  wharf  grind  against  one  another  with  prodigious  force,  by 
the  slow  movement  of  the  tide.— Soldiers  have  thought  to  stop  a  spent  cannon-ball  by 
putting  a  foot  against  it,  but  have  found  its  momentum  sufficient  to  break  a  leg. 

On  the  other  hand,  a  light  body  moving  with  a  high  velocity  may  have  an  enor- 
mous momentum.  Ex. :  The  air  in  a  hurricane  will  tear  up  trees  by  the  roots  and 
level  buildings  to  the  ground.— Sand  driven  from  a  tube  by  steam  is  used  for  drilling, 
and  in  stone-cutting,  engraving,  etc. 

*  A  stone  thrown  against  a  pane  of  glass  shatters  it ;  but  a  bullet  fired  through  it 
will  make  only  a  round  hole.  The  bullet  is  gone  before  the  motion  has  time  to  past< 
into  the  surrounding  particles. — A  fraction  of  time  is  required  for  a  ball  to  receive 
the  force  of  the  exploding  powder  and  to  get  under  full  headway.  An  instrument  is 
used  to  determine  the  acceleration  of  speed  before  leaving  the  gun. 


LAWS   OF   MOTION. 

To  this  law  are  to  be  referred  many  ordinary  illustrations 
of  the  so-called  "inertia  of  matter."  Thus,  when  we  en- 
deavor to  stop  a  moving  body,  as  a  wagon,  we  must  over- 
come its  momentum.  The  danger  in  jumping  from  a  car  in 
rapid  motion  lies  in  the  fact  that  the  body  has  the  speed  of 
the  train,  while  the  forward  motion  of  the  feet  is  checked 
by  the  contact  with  the  ground.* 

» 

SECOND  LAW. — A  force  acting  upon  a  lody  in  motion  or  at 
rest,  produces  the  same  effect  whether  it  acts  alone  or  with 
other  forces.  Ex.  :  All  bodies  upon  the  earth  are  in  constant 
motion  with  it,  yet  we  act  with  the  same  ease  that  we  should 
were  the  earth  at  rest.f — We  throw  a  stone  directly  at  an 
object  and  hit  it,  yet,  within  the 
second,  the  mark  has  gone  for- 
ward many  feet.J — If  a  cannon- 
ball  be  thrown  horizontally,  it  will 
fall  as  fast  and  strike  the  earth  as 
soon  as  if  dropped  to  the  ground 
from  the  muzzle  of  the  gun.  In 
Fig.  6,  D  is  an  arm  driven  by  a 
wooden  spring,  E,  and  turning  on  a  hinge  at  C.  At  D  is  a 
hollow  containing  a  bullet,  so  placed  that  when  the  arm  is 
sprung,  the  ball  will  be  thrown  in  the  line  FK.  At  F  is  a 

*  Some  jnmp  as  nearly  ua  possible  in  the  direction  in  which  the  train  is  moving, 
ami  are  ready  to  run  the  instant  their  feet  touch  the  ground.  Then  with  all  their 
strength  they  gradually  overcome  the  inertia  of  the  body,  and  after  a  few  rods  can 
turn  as  they  please.  If  one  could  jump  backward  with  sufficient  force  to  overcome 
t  he  forward  motion  of  the  train,  it  would  then  be  possible  to  drop  directly  downward. 

t  A  ball  thrown  up  into  the  air  with  a  force  that  would  cause  it  to  rise  50  feet,  will 
ascend  to  that  height  whatever  horizontal  wind  may  be  blowing.— While  riding  on  a 
car,  we  throw  a  stone  at  some  object  at  rest.  The  stone,  having  the  motion  of  the 
train,  strikes  just  as  far  ahead  of  the  object  as  it  would  have  gone  had  it  remained  on 
the  train.  In  order  to  hit  the  mark,  we  should  have  aimed  a  little  back  of  it.— The 
circus-rider  wishes,  while  his  horse  is  at  full  speed,  to  jump  through  a  hoop  suspend- 
ed before  him.  He  simply  springs  directly  upward.  Going  forward  by  the  momen- 
tum which  he  had  acquired  before  he  leaped  from  the  horse,  he  passes  through  the 
hoop  and  alights  upon  the  saddle  again. — A  person  riding  in  a  coach  drops  a  cent  to 
the  floor.  It  apparently  t-trikes  where  it  would  if  the  coach  were  at  rest. 

t  The  earth  moves  in  its  orbit  around  the  sun  at  the  rate  of  about  18  miles  per 
second.  See  Fourteen  Weeks  in  Astronomy,  p.  106. 


30  MOTIOK    AND    FOBOE. 

similar  ball,  supported  by  a  thin  slat,  G,  and  so  arranged 
that  the  same  blow  which  throws  the  ball  D,  will  let  the  bail 
F  fall  in  the  line  FH.  The  two  balls  will  strike  the  floor  at 
the  same  instant. 

THIED  LAW. — Action  is  equal  to  reaction,  and  in  the  con- 
trary  direction.  Ex.  :  A  bird  in  flying  beats  the  air  down- 
ward, but  the  air  reacts  and  supports  the  bird. — The  powder 
in  a  gun  explodes  with  equal  force  in  every  direction,  driving 
the  gun  backward  and  the  ball  forward,  with  the  same 
momentum.  Their  velocities  vary  with  their  weights  ;  the 
heavier  the  gun,  the  less  will  the  recoil  be  noticed. — When 
we  spring  from  a  boat,  unless  we  are  cautious,  the  reaction 
will  drive  it  from  the  shore.— rWhen  we  jump  from  the 
ground,  we  push  the  earth  from  us,  while  it  reacts  and 
pushes  us  from  it ;  we  separate  from  each  other  with  equal 
momentum,  and  our  velocity  is  as  much  greater  than  that  of 
the  earth  as  we  are  lighter. — We  walk,  therefore,  by  reason 
of  the  reaction  of  the  ground  on  which  we 
tread. 

The  apparatus  shown  in  Fig.  7  consists  of 
ivory  balls  hung  so  as  readily  to  vibrate.*  If 
a  ball  be  let  fall  from  one  side,  it  will  strike 
the  second  ball,  which  will  react  with  an  equal 
force,  and  stop  the  motion  of  the  first,  but  transmit  the 
motion  to  the  third  ;  this  will  act  in  the  same  manner,  and 
BO  on  through  the  series,  each  acting  and  reactirig  until  the 
last  ball  is  reached ;  this  will  react  and  then  bound  off, 
rising  as  high  as  the  first  ball  fell  (except  the  loss  caused  by 
resistances  to  motion).  If  two  balls  be  raised,  two  will  fly 
off  at  the  opposite  end ;  if  two  be  let  fall  from  one  side  and 
rone  from  the  other,  they  will  respond  alternately. 

6.   Compound  Motion.— Let  a  ball  at  A  (Fig.  8)  be 
acted  on  by  a  force  which  would  drive  it  in  a  given  time  to 

*  The  same  experiments  can  be  performed  by  means  of  glass  marbles  or  billiard 
balls  placed  in  a  groove.  Better  still,  attach  strings  to  glass  marbles  by  means  of 
mucilage  and  bits  of  paper  and  suspend  them  from  a  simple  wooden  frame. 


COMPOUND     MOTIOK. 

B,  and  also  at  the  same  instant  by 
another  which  would  drive  it  to  D  in 
the  same  time ;  the  ball  will  move  in 
the  direction  AC.  Ex.  :  A  person 
wishes  to  row  a  boat  across  a  swift  cur- 
rent which  would  carry  him  down  stream.  He  therefore 
steers  toward  a  point  above  that  which  he  wishes  to  reach, 
and  so  goes  directly  across. — A  bird,  beating  the  air  with 
both  its  wings,  flies  in  a  direction  different  from  that  which 

would  be  given  by  either 

one. 

7.  Composition  of 
Forces. — When  a  body 
is  thus  acted  on  by 
two  forces,  we  draw 
lines  representing  their 
directions,  and  mark 
off  AD  and  AB,  whose 
lengths  represent  their 
comparative  mag- 
nitudes. We  next  com- 
plete the  parallelogram 
and  draw  the  diagonal 

AC,  which  denotes  the  resultant  of  these  forces,  and  gives 
the  direction  in  which  the  body  will  move.  If  more  than 
two  forces  act,  we  find  the  resultant  of  two,  then  of  that 
resultant  and  a  third  force,  and 

so  on.  ftfcW- 

X 

8.  Resolution  of  Forces  con- 
sists in  finding  what  two  forces  are 
equivalent  to  a  given  force.  A  par- 
allelogram is  drawn  having  the  given  w 
force  as  a  diagonal.  Ex.  :  There  is 
a  wind  blowing  from  the  west  against 
GH  (Pig.  10),  the  sail  of  a  vessel 
going  north.  We  can  resolve  the 


MOTION    AND    FORCE. 


Fie.  It 


FIG.  13. 
V 


wind-force  BD  into  the  two  forces  BE  and  BC.  The  former, 
blowing  parallel  to  the  sail,  is  of  no  use ; 
the  latter  is  perpendicular  to  it,  and  drives 
the  vessel  northeast.  Again,  resolving  BD 
in  Fig.  11,  which  represents  the  vertical 
force  BO  in  Fig.  10,  we  find  that  it  is 
equivalent  to  two  forces  BE  and  BC.  The 
former  pushes  the  vessel  sideways,  but  is 
mainly  counteracted  by  the  shape  of  the 
keel  and  the  action  of  the  rudder.  The 
latter  is  parallel  to  the  course  of  the  ship, 
and  hurries  it  north. 

By  shifting  the  rigging,  one  vessel  will  sail  into  harbor 
while  another  is  sail- 
ing out,  both  driven 
by  the  same  wind.* 
Figs.  12  and  13  show 
how,  by  twice  resolv- 
ing the  force  of  the  w. 
wind  from  the  W.,  as 
in  the  last  figures, 
when  the  sail  GH  is 
placed  in  the  new  po- 
sition, we  have  (Fig. 
13)  a  force  BO,  which  drives  the  vessel  S.f  If  a  vessel 

*  The  toy  shown  in  Pig.  14,  and  easily  made  by  any 
pnpil,  proves  how  a  change  in  the  position  of  the  sails 
will  produce  a  contrary  effect.  Carry  this  wind-mill 
forward,  and  the  two  sets  of  feather -vanes  will  revolve 
swiftly  in  opposite  directions. 

t  In  a  similar  manner  we  may  resolve  the  three  forces 
which  act  upon  a  kite— viz.,  the  pull  of  the  string,  the 
force  of  the  wind,  and  its  own  weight.  In  Fig.  10,  let 
GH  represent  the  face  of  the  kite.  We  can  resolve  BD, 
the  force  of  the  wind,  into  BC  and  BE.  We  next  resolve 
BD,  in  Fig.  11,  which  corresponds  to  BC  in  Fig.  10  into 
BE  and  BC.  We  then  have  a  force,  BC,  which  overcomes 
the  weight  of  the  kite  and  also  tends  to  lift  it  upward. 
The  string  pulls  in  the  direction  BD,  perpendicularly  to 
the  face.  The  kite  obeys  both  of  these  forces,  and  so 
ascends  in  a  direction  DG,  between  the  two.  It  is  really 
drawn  up  an  inclined  plane  by  the  joint  force  of  the  wind  and  the  string. 


CIRCULAR    MOTIOK.  33 

were  to  He  sailed  due  W.  against  the  wind,  it  would  tack 
alternately  K  W.  and  SW.  In  this  way  it  could  go  almost  in 
the  "teeth  of  the  wind." 

A  canal-boat  drawn  by  horses  is  acted  upon  by  a  force 
which  tends  to  bring  it  to  the  bank.  This  force  may  be 
resolved  into  two,  one  pulling  toward  the  tow-path,  and  the 
other  directly  ahead.  The  former  is  counteracted  by  the 
shape  of  the  boat  and  the  action  of  the  rudder ;  the  latter 
draws  the  boat  forward. 

9.  Motion  in  a  Curve.- — Whenever  two  or  more  in- 
stantaneous forces  act  upon  a  body,  the  path  is  a  straight 
line.     When  one  is  instantaneous  and  the  other  continuous, 
it  is  a  curved  line.     Ex.  :  When  a  body  is  thrown  into  the 
air,  except  in  a  vertical  line  (p.  54),  it  is  acted  upon  by  the 
instantaneous  force  of  projection  and  the  continuous  force 
of  gravity,  and  so  describes  a  line  which  curves  toward  the 
earth. 

10.  Circular  Motion  is  produced  when  a  moving  body 
is  drawn  toward  a  centre  by  a  constant  force.     Thus,  when 
a  sling  is  whirled,  the  stone  is  pulled  toward  the  hand  by 
the  string,  and  as,  according  to  the  third  law  of  motion, 
every  action  has  its  equal  and  opposite  reaction,  the  hand  is 
pulled  toward  the  stone.     If  the  string  break,  the  stone  will 
continue  to  move,  according  to  the  first  law  of  motion,  in 
a  straight  line  in  the  direction  of  a  tangent  to  the  circle  at 
that  point.      The  tension  of  the  string,  acting  inward,  is 
called  the  Centripetal  {centrum,  the  centre,  peter e,  to  seek) 
force  ;  and  the  reaction  of  the  stone^upon  the  string,  acting 
outward,  is  termed  the   Centrifugal  (centrum,  the  centre, 
fugere,  to  flee)  force.* 

*  It  should  be  noticed  that  in  circular  motion  there  is  bat  one  true  force  concerned. 
It  acts,  however,  upon  a  body  in  motion.  The  so-called  centrifugal  force  has  nothing 
to  do  with  the  production  of  the  motion,  being  merely  the  resistance  which  the  body 
offers  by  its  inertia  to  the  operation  of  the  centripetal  force,  and  ceases  the  instant 
that  force  is  discontinued.  It  does  not  act  at  right  angles  to  the  centripetal  force,  as 
is  often  stated,  but  in  direct  opposition.  A  body  never  flies  off  from  the  centre  im- 
pelled by  the  centrifugal  force,  since  that  can  never  exceed  the  centripetal  (action  = 
reaction),  and  moreover  the  path  of  snch  a  body  is  in  the  direction  of  a  tangent,  and 


34  MOTION    AND    FORCE. 

The  following  examples  are  among  those  usually  given 
to  illustrate  the  action  of  the  centre-fleeing  force  :  Water 
flies  from  a  grindstone  on  account  of  the  centrifugal 
force  produced  in  the  rapid  revolution,  which  overcomes 
the  adhesion. — In  factories,  grindstones  are  sometimes  re- 
volved with  such  velocity  that  this  force  overcomes  that 
of  cohesion,  and  the  ponderous  stones  fly  into  fragments. — 
A  pail  full  of  water  may  be  whirled  around  so  rapidly  that 
none  will  spill  out,  because  the  centrifugal  force  overcomes 
that  of  gravity. — When  a  horse  is  running  around  a  small 
circle,  he  bends  inward  to  overcome  the  centrifugal  force. 

The  heavenly 
bodies  present  the 
-o —  grandest  example 
of  circular  motion. 
We  may  suppose  the 
earth  to  have  been 
moving  originally 
in  the  direction  AC. 
The  attraction  of 
the  sun,  however, 
drawing  it  in  the 
direction  BS,  it  passes  along  the  line  BD.  If  the  centripe- 
tal force  were  suddenly  to  cease,  the  earth  would  fly  oif 
into  space  along  a  tangent,  as  BO.  The  rapid  revolution  of 
the  earth  on  its  axis  tends  to  throw  off  all  bodies  headlong. 
Ac  this  acts  in  opposition  to  gravity,  it  diminishes  the  weight 

not  th«*  radius  of  a  circle.  Thus,  when  water  is  thrown  off  a  grindstone  in  rapid 
rotation,  th-*  tendency  of  the  water  to  continue  to  move  on  in  the  direction  of  the 
straight  line  in  which  it  is  going  at  each  instant  (in  other  words,  the  inertia  of  the 
water)  overcomes  its  adhesion  to  the  stone,  and  it  flies  off  in  obedience  to  the  first 
law  of  motion.  So,  also,  when  a  grindstone,  driven  at  a  high  speed,  breaks,  and  the 
fragments  are  thrown  with  great  velocity,  we  are  not  to  suppose  that  the  centrifugal 
fore'  impels  them  through  the  air.  That  force  existed  only  while  the  stone  was 
entire.  It  was  opposed  to  the  force  of  cohesion,  and  in  the  moment  of  its  triumph 
ceased,  and  the  fragments  of  the  stone  fly  off  in  virtue  of  the  velocity  they  possess 
at  thai  instant  Again  the  so-called  centrifugal  force  is  not  a  real  force  urging  bodies 
upward  at  the  equator.  The  earth's  surface  is  merely  falling  away  from  a  tangent, 
and  a  part  of  the  force  of  gravity  is  spent  in  overcoming  the  inertia  of  bodies.  The 
ter.  u  centrifugal  force  has  caused  much  confusion,  and  will  doubtless  soon  be  dis- 
carded. 


CIKCULAR    MOTION. 


35 


FIG.  16. 


FIG.  17. 


of  bodies  at  the  equator,  where  it  is  greatest,  -pfa.  It  also 
tends  to  drive  the  water  on  the  earth 
from  the  poles  toward  the  equator. 
Were  the  velocity  of  the  earth's  ro- 
tation to  diminish,  the  water  would 
run  back  toward  the  poles,  and  tend 
to  restore  the  earth  to  a  spherical 
form.  This  influence  is  well  illus- 
trated by  the  apparatus  shown  in 
Fig.  16.  The  hoop  is  made  to  slide 
upon  its  axis,  and  if  revolved  rapidly 
it  will  assume  an  oval  form,  bulging 
out  more  and  more  as  the  velocity  is 
increased.* 

11.  The  Gyroscope  beauti- 
fully illustrates  the  principle  of  the 
composition  of  forces  in  rotary 
motion.  In  Fig.  17  a  wheel  re- 
volves within  a  ring  which  is  sus- 
tained at  one  end  by  an  upright 
support.  If  the  wheel  is  made  to 
revolve  swiftly  by  unwinding  a 
string,  and  then  placed  on  the  sup- 
port, instead  of  falling,  as  one 
would  suppose,  the  whole  begins 
to  revolve  rapidly  around  the  point  of  support,  in  a  resultant 
between  the  force  of  gravity  and  the  rotary  motion  of  the 
wheel.  If  we  attempt  to  raise  or  lower  the  ring,  it  will  sen- 
sibly oppose  the  change  and  persist  in  its 
plane  of  rotation. 

12.  Reflected  Motion  is  produced  by 
the  reaction  of  a  surface  against  which  an 
elastic  body  is  cast.  If  a  ball  be  thrown 

*  This  apparatus  is  accompanied  by  objects  to  illustrate  the  principle  that  all 
bodies  tend  to  revolve  about  their  shortest  diameters,  an  assurance  that  the  earth 
will  never  change  its  axis  of  rotation  while  it  retains  its  present  form.  "  Tie  to 


FIG.  18. 


36  MOTION    AND    fORCE. 

in  the  direction  OB  against  the  surface  AC,  it  will  rebound  in 
the  line  BE.  The  angle  OBP,  that  si  incidence,  =  the 
angle  PBR,  that  of  reflection. 

13.  Energy  is  the  power  of-  doing  work,  i.  e.9  of  over- 
coming any  kind  of  resistance.     It  is  in  general  something 
put  into  a  body  by  means  of  work,  and  which  comes  out  of 
it  when  it  does  work.     Ex.  :  A  wound-up  clock,  a  red-hot 
iron.     The   difference  between  energy  and  momentum  is 
evident.     When  a  bullet  is  fired  from  a  rifle,  the  momenta 
of  both  are  equal,  but  the  energy  of  the  former,  i.  e.,  its 
power  of  doing  work,  as  piercing  a  board,  is  far  greater. 
Energy  is  proportional  to  the  square  of  the  velocity.     Thus, 
a  cannon-ball  given  double  speed  will  penetrate  four  times 
as  far  into  a  wall ;  and  a  stone  thrown  upward  at  the  rate  of 
96  feet  per  second  will  rise  9  times  as  far  as  with  a  velocity 
of  32  feet  (p.  55). 

14.  Two  Forms  of  Energy. — Energy  may  be  either 
active  or  latent.     When  a  rock  is  tumbling  down  a  moun- 
tain-side, it  exhibits  the  force  of  gravity  in  full  sway ;  but 
when  the  rock  was  lodged  on  the  mountain-top,  it  possessed 
the  same  energy,  which  could  be  developed  at  any  moment 
by  loosening  it  from  its  place.     These  two  forms  are  known 


the  middle  of  a  lead  pencil  a  piece  of  string  about  three  feet  long.  Suspend  so 
that  the  pencil  will  balance  itself.  Now  twist  the  end  of  the  string  between  the 
thumb  and  the  first  finger  of  the  right  hand,  steadying  and  holding  the  string  with 
the  left  hand.  A  circular  motion  will  thus  be  communicated  to  the  pencil,  and  it 
will  revolve  around  the  point  on  which  it  is  suspended.  Tie  a  piece  of  white  string 
around  the  middle  of  the  pencil,  or  its  centre  of  gravity,  simply  to  show  the  position 
of  that  point.  Now  tie  the  first  piece  of  string  half-way  between  the  end  of  the 
pencil  and  the 'centre  of  gravity,  an  J  communicate  the  circular  motion  described 
above,  and  we  shall  observe  that  the  pencil  will  still  revolve  around  the  centre  of 
gravity,  the  point  marked  by  the  white  string  being  at  rest.  It  can  thus  be  shown 
that  anything,  of  whatever  shape,  will  tend  to  revolve  on  its  shortest  diameter.  If 
the  end  links  of  a  small  steel  chain  (such  as  is  often  attached  to  purses  or  parasols) 
be  hooked  together,  the  string  tied  to  a  link,  and  the  circular  motion  given,  it  will 
be  observed  that  the  chain  begins  to  take  an  elliptical  form,  which  gradually 
approaches  that  of  a  circle,  until  at  last  it  becomes  a  circle,  when  it  revolves 
horizontally.  This  shows  that  even  a  ring  is  subject  to  the  same  law— that  is, 
revolves  on  its  shortest  axis." 


ENERGY.  37 

as  energy  of  motion  and  energy  of  position,  or  actual  and 
potential  (possible)  energy.* 

15.  Conservation  of  Energy.— One  kind  of  energy  is 
changed  into  another  without  loss.  The  sum  of  all  the 
energy  in  the  universe  remains  the  same.  A  hammer  falls 
by  the  force  of  gravity  and  comes  to  rest.  Its  potential 
energy  changes  to  kinetic  and  does  work.  Its  motion  as  a 
mass  is  converted  into  one  of  atoms,  and  reveals  itself  to  our 
touch  as  heat  (p.  184).  f 

PRACTICAL  QUESTIONS.— 1.  Can  a  rifle-ball  be  fired  through  a  handkerchief 
suspended  loosely  from  one  corner  ?  2.  A  rifle-ball  thrown  against  a  board  standing 
edgewise,  will  knock  it  down  ;  the  same  bullet  fired  at  the  board  will  pass  through 
it  without  disturbing  its  position.  Why  is  this  ?  3.  Why  can  a  boy  skate  safely 
over  a  piece  of  thin  ice,  when,  if  he  should  pause,  it  would  break  under  him  directly. 
4.  Why  can  a  cannon-ball  be  fired  through  a  door  standing  ajar,  without  moving  it 
on  its  hinges  ?  5.  Why  can  we  drive  on  the  head  of  a  hammer  by  simply  striking  the 
end  of  the  handle  ?  6.  Suppose  yon  were  on  a  train  of  cars  moving  at  the  rate  of  30 
miles  per  hour;  with  what  velocity  would  you  be  thrown  forward  if  the  train  were 

*  Actual  energy  is  also  styled  dynamic  or  kinetic  energy,  and  potential  is  termed 
static  energy.  In  mechanics,  kinetic  energy  is  called  vis  viva  (=  ^mv2),  or  striking 
force.  We  wind  a  watch,  and  by  a  few  moments  of  labor  condense  in  the  spring  a 
potential  energy,  which  is  doled  out  for  24  hours  in  the  dynamic  energy  of  the  wheels 
and  hands.  Draw  a  violin  bow,  and  the  potential  energy  of  the  arm  is  stored  up  in 
the  stretched  cord.  Lift  a  pendulum,  and  you  thereby  give  the  weight  potential 
energy.  Let  it  fall,  and  the  potential  changes  gradually  to  dynamic.  At  the  centre 
of  the  arc  the  potential  is  gone  and  kinetic  is  possessed.  Then  the  kinetic  changes 
again  to  potential,  which  increases  till  the  end  of  the  arc  is  reached  and  the  pen- 
dulum ceases  to  rise,  when  the  energy  is  that  of  position,  not  of  motion.  Potential 
energy  is  one  that  is  concealed,  lying  in  wait  and  ready  to  burst  forth  on  the  instant. 
It  is  a  loaded  gun  prepared  for  the  arm  of  the  marksman.  It  is  a  river  trembling  on 
the  brink  of  a  precipice,  about  to  take  the  fearful  leap.  It  is  a  weight  wound  up  and 
held  against  the  tug  of  gravity.  It  is  the  engine  on  the  track  with  the  steam  hissing 
from  every  crevice.  It  is  the  drop  of  water  with  a  thunderbolt  hidden  within  its 
crystal  walls.  On  the  contrary,  dynamic  energy  is  in  full  view,  in  actual  operation. 
The  bullet  is  speeding  to  the  mark ;  the  river  is  tumbling ;  the  weight  is  falling ;  the 
engine  is  flying  over  the  rails  ;  and  the  bolt  is  flashing  across  the  sky.  It  is  heat 
radiating  from  our  fires  ;  electricity  flashing  our  messages  over  the  continent ;  and 
gravity  drawing  bodies  headlong  to  the  earth. 

t  No  energy  in  nature  can  be  wasted.  It  must  accomplish  something.  "  A  blow 
with  a  hammer  moves  the  earth.  A  boy  could  in  time  draw  the  largest  ship  across 
the  harbor  in  calm  weather." 

"  Water  falling  day  by  day 

Wears  the  hardest  rock  away." 

Statues  are  worn  smooth  by  the  constant  kissing  of  enthusiastic  worshippers. 
Stone  steps  are  hollowed  by  the  friction  of  many  feet.  The  ocean  is  filled  by  small 
drops  which  fall  from  the  clouds.  We  may  notice  none  of  these  forces  singly,  but 
their  effects  in  the  aggregate  startle  us. 


38  MOTION    AND    FORCE. 


stopped  Instantly  ?  7.  In  what  line  does  a  stone  fall  from  the  masthead  of  a  vessel  in 
motion  ?  8.  If  a  ball  be  dropped  from  a  high  tower,  it  will  strike  the  ground  a  little 
qast  of  a  vertical  line.  Why  is  this  ?  9.  It  is  stated  that  a  suit  was  once  brought  by 
the  driver  of  a  light  wagon  against  the  owner  of  a  coach  for  damages  caused  by  a  col- 
lision. The  complaint  was  "  the  latter  was  driving  so  fast  that  when  the  two  carriages 
struck,  the  driver  of  the  former  was  thrown  forward  over  the  dashboard."  On  trial  he 
was  nonsuited,  because  his  own  evidence  showed  him  to  be  the  one  who  was  driving 
at  the  unusual  speed.  Explain.  10.  Suppose  a  train  moving  at  the  rate  -of  30  miles 
per  hour  ;  on  the  rear  platform  is  a  spring-gun  aimed  parallel  to  the  track  and  in  a 
direction  precisely  opposite  to  the  motion  of  the  car.  Let  a  ball  be  discharged  with 
the  exact  speed  of  the  train  ;  where  would  it  fall  ?  11.  Suppose  a  steamer  in  rapid 
motion,  and  on  its  deck  a  man  jumping.  Can  he  jump  further  by  leaping  the  way 
the  boat  is  moving  than  in  the  opposite  direction  ?  13.  If  a  stone  be  dropped 
from  the  masthead  of  a  vessel  in  motion,  will  it  strike  the  same  spot  on  the  deck 
that  it  would  if  the  vessel  were  at  rest  ?  14.  Could  a  party  play  ball  on  the  deck  of 
the  Great  Eastern  when  steaming  along  at  the  rate  of  20  miles  per  hour,  without 
making  allowance  for  the  motion  of  the  ship  ?  15.  Since  action  is  equal  to  reaction, 
why  is  it  not  so  dangerous  to  receive  the  "  kick"  of  a  gun  as  the  force  of  the  bullet  ? 
16.  If  you  were  to  jump  from  a  carriage  in  rapid  motion,  would  you  leap  directly 
toward  tvte  spot  on  which  you  wished  to  alight  ?  17.  If  you  wished  to  shoot  a  bird 
in  swift  flight,  would  you  aim  directly  at  it  ?  18.  At  what  parts  of  the  earth  is  the 
centrifugal  force  least  ?  19.  What  causes  the  mud  to  fly  from  the  wheels  of  a  carriage 
in  rapid  motion  ?  20.  What  proof  have  we  that  the  earth  was  once  a  soft  mass  ? 
81.  On  a  curve  in  a  railroad,  one  track  is  always  higher  than  the  other.  Why  is  this  ? 
22.  What  is  the  principle  of  the  sling?  23.  The  mouth  of  the  Mississippi  River  is 
about  2£  miles  farther  from  the  centre  of  the  earth  than  its  source.  In  this  sense  it 
may  be  said  to  "  run  up  hill."  What  causes  this  apparent  opposition  to  the  attrac- 
tion of  gravity  ?  24.  Is  it  action  or  reaction  that  breaks  an  egg,  when  I  strike  it 
against  the  table  ?  25.  Was  the  man  philosophical  who  said  that  it  "  was  not  the 
falling  so  far,  but  the  stopping  so  quick,  that  hurt  him?"  26.  If  one  person  runs 
against  another,  which  receives  the  greater  blow  ?  27.  Would  it  vary  the  effect  if  the 
two  persons  were  running  in  opposite  directions  ?  In  the  same  direction  ?  28.  Why 
can  you  not  fire  a  rifle-ball  around  a  hill?  29.  Why  is  it  that  a  heavy  rifle  "kicks" 
less  than  a  light  shot-gun  ?  30.  A  man  on  the  deck  of  a  large  vessel  draws  a  small 
boat  toward  him.  How  much  does  the  ship  move  to  meet  the  boat?  31.  Suppose  a 
string,  fastened  at  one  end,  will  just  support  a  weight  of  25  Ibs.  at  the  other.  Unfas- 
ten it,  and  let  two  persons  pull  upon  it  in  opposite  directions.  How  much  can  each 
pull  without  breaking  it  ?  32.  Can  a  man  standing  on  a  platform-scale  make  himself 
lighter  by  lifting  up  on  himself?  33.  Why  cannot  a  man  lift  himself  by  pulling  up 
on  his  boot-straps  ?  34.  If,  from  a  gun  placed  vertically,  a  ball  were  fired  into  per- 
fectly still  air,  where  would  it  fall  ?  35.  With  what  momentum  would  a  steamboat 
weighing  1,000  tons,  and  moving  with  a  velocity  of  10  feet  per  second,  strike  against 
a  sunken  rock?  36.  With  what  momentum  would  a  train  of  cars  weighing  100  tons, 
and  running  10  miles  per  hour,  strike  against  an  obstacle  ?  37.  What  would  be  the 
comparative  striking  force  of  two  hammers,  one  driven  with  a  velocity  of  20  feet  per 
second  and  the  other  10  feet  ?  38.  If  a  100  horse-power  engine  can  propel  a  steamer 
5  miles  per  hoar,  will  one  of  200  horse-power  double  its  speed  T  39.  Why  is  a  bullet 
flattened  if  fired  obliquely  against  the  surface  of  water  ?  40.  Why  are  ships  becalmed 
at  sea  often  floated  by  strong  currents  into  dangerous  localities  without  the  knowl- 
edge of  the  crew  ?  41.  A  man  in  a  wagon  holds  a  50-lb.  weight  in  his  hand.  Suddenly 
the  wagon  falls  over  a  precipice.  Will  he,  while  dropping,  bear  the  strain  of  the 
weight?  42.  Why  are  we  not  sensible  of  the  rapid  motion  of  the  earth?  43.  A 
feather  is  dropped  from  a  balloon  whict  is  immersed  in  and  swept  along  by  a  swift 
current  of  air.  Will  the  feather  be  blown  away  or  will  it  appear  to  drop  directly 


SUMMABT.  39 

down  f  44.  Suppose  a  bomb-shell,  flying  through  the  air  at  the  rate  of  600  feet  per 
second,  explodes  into  two  parts  ot  equal  weight,  driving  one-half  forward  in  the  same 
direction  as  before,  but  with  double  its  former  velocity.  What  would  become  of  the 
other  half?  45.  Which  would  have  the  greater  penetrating  power,  a  small  cannon- 
ball  with  a  high  velocity,  or  a  large  one  with  a  low  velocity  ?  46.  There  is  a  story 
told  of  a  man  who  erected  a  huge  pair  of  bellows  in  the  stern  of  his  pleasure-boat, 
that  he  might  always  have  a  fair  wind.  On  trial,  the  plan  failed.  In  which  direction 
should  he  have  turned  the  bellows  ?  47.  If  a  man  and  a  boy  were  riding  in  a  wagon, 
and,  on  coming  to  the  foot  of  a  hill,  the  man  should  take  up  the  boy  in  his  arms, 
would  that  help  the  horse  ?  48.  Why  does  a  bird,  as  it  begins  to  fly,  always,  if  possi- 
ble, turn  toward  the  wind  ?  49.  If  we  whirl  a  pail  of  water  swiftly  around  with  our 
hands,  why  will  the  water  tend  to  leave  the  centre  of  the  pail  ?  50.  Why  w  ill  the  foam 
collect  at  the  hollow  in  the  centre  ?  51.  If  two  cannon-balls,  one  weighing  8  Ibs.  and 
the  other  2  Ibs.,  be  fired  with  the  same  velocity,  which  will  go  the  further  ?  52.  Re- 
solve the  force  of  the  wind  which  turns  a  common  wind-mill,  and  show  how  one  part 
acts  to  push  the  wheel  against  its  support,  and  one  to  turn  it  around.  63.  Why  is  a 
gun  firing  blank  cartridges  more  quickly  heated  than  one  firing  balls  ?  54.  When  an 
animal  is  jumping  or  falling,  can  any  exertion  made  in  mid-air  change  the  motion  of 
its  centre  of  gravity  ?  55.  If  one  is  riding  rapidly,  in  which  direct  ion  will  he  be  thrown 
when  the  horse  is  suddenly  stopped  ?  58.  When  standing  in  a  boat,  why,  as  it  starts, 
are  we  thrown  backward  ?  59.  When  carrying  a  cup  of  tea,  if  we  move  or  stop 
quickly,  why  is  the  liquid  liable  to  spill?  58.  Why,  when  closely  pursued,  can  we 
escape  by  dodging  ?  59.  Why  is  a  carriage  or  sleigh,  when  sharply  turning  a  corner, 
liable  to  tip  over  ?  60.  Why,  if  you  place  a  card  on  your  finger  and  on  top  of  it  a 
cent,  can  you  snap  the  card  from  under  the  cent,  which  will  then  drop  on  your  finger? 
61.  Why  is  a  "  running  jump"  longer  than  a  "  standing  jump"  ?  62.  Why,  after  the 
sail?  of  a  vessel  are  furled,  does  it  still  continue  to  move  ?  and  why,  after  the  sails  are 
spread,  does  it  require  some  tune  to  get  it  under  full  headway  ?  63.  Why  can  a  tal- 
low candle  be  fired  through  a  board  ? 


SUMMARY. 

Matter,  so  far  as  we  know  it,  is  in  constant  change.  This  change 
of  place  is  termed  motion.  Terrestrial  motion  is  restricted  by  friction, 
by  the  air,  and  by  water.  Friction  is  caused  by  the  roughness  of  the 
surface  over  which  a  body  moves.  It  may  be  decreased  by  the  use  of 
grease  to  fill  up  the  minute  projections,  or  by  changing  the  sliding 
into  rolling  friction.  Air  and  water  must  be  displaced  by  a  moving 
body,  and  the  resistance  they  offer  is  increased,  in  general,  according 
to  the  square  of  its  velocity.  Motion  is  governed  by  three  laws  ;  viz.: 
A  moving  body  left  to  itself  tends  to  go  forever  in  a  straight  line  ;  a 
force  has  the  same  effect  whether  it,  acts  alone  or  with  other  forces,  and 
upon  a  body  at  rest  or  in  motion  ;  and  action  is  equal  and  opposed  to 
reaction.  By  means  of  the  principles  of  the  composition  and  resolu- 
tion of  forces,  we  can  find  the  individual  effect  of  a  single  force  or  the 
combined  effect  of  several  forces.  Motion  produced  by  two  or  more 
instantaneous  forces  is  in  a  straight  line  ;  when  one  is  continuous, 


40  MOTION    AND    FORCE. 

the  result  is  a  curved  line ;  and  when  the  continuous  force,  directed 
toward  a  fixed  point,  acts  upon  a  moving  body,  a  circle  is  then 
described.  A  croquet  ball  struck  by  two  mallets  at  the  same  moment, 
illustrates  the  first  kind  of  motion ;  the  path  of  a  bullet  or  rocket  in 
the  air  exhibits  the  second  ;  and  the  movement  of  a  stone  whirled  in  a 
sling  is  an  example  of  the  third.  When  a  rubber  ball  bounds  back 
from  a  surface  against  which  it  is  thrown,  the  angle  of  reflection  equals 
the  angle  of  incidence. 

Energy,  or  the  power  of  doing  work,  is  a  general  term  employed  to 
unify  all  the  forces  of  nature.  Out  of  it  grows  the  grand  law  of  the 
Conservation  of  Energy,  which  teaches  that  the  different  forces  are 
only  different  forms  of  one  all-pervading  energy,  and  that  they  are 
mutually  interchangeable,  and  indestructible  as  matter  itself. 


HISTORICAL    SKETCH. 

Aristotle  taught  that  all  motion  is  naturally  circular,  and  this  view 
was  held  by  his  school.  He  divided  the  phenomena  of  motion  into  two 
classes — the  natural  and  the  violent.  As  an  instance  of  the  former,  he 
gave  the  falling  of  a  stone,  which  constantly  increases  in  velocity ;  and 
of  the  latter,  a  stone  thrown  vertically  up,  which  being  against  nature, 
continually  goes  slower.  Newton,^  in  his  "  Principia "  published  in 
1687,  propounded  the  laws  of  motion  as  now  received.  Other  philoso- 
phers, notably  Galileo,  Hooke,  and  Huygens,  had  anticipated  much 
of  his  reasoning,  yet  so  slowly  were  his  opinions  accepted  that  "  at  his 
death,"  says  Voltaire,  "he  had  not  more  than  twenty  followers  outside 
of  England." 

The  law  of  the  Conservation  of  Energy,  Faraday,  the  great  Eng- 
lish physicist,  pronounced  "  the  grandest  ever  presented  for  the  con- 
templation of  the  human  mind/*  It  has  been  established  within  the 
present  century ;  yet  we  now  know  that  former  scholars  had  inklings 
of  the  wonderful  truth.  It  arose  in  connection  with  discoveries  on  the 
subject  of  Heat,  and  its  history  will  be  treated  of  hereafter. 

Consult  Stewart's  "Conservation  of  Energy";  Youmans's  "Cor- 
relation of  the  Physical  Forces";  Faraday's  "  Lectures  on  the  Phys- 
ical Forces";  Everett's  "  Deschanel's  Natural  Philosophy";  Tait's 
"Recent  Advances  in  Physical  Science";  Maxwell's  "Matter  and 
Motion";  "Appleton's  Cyclopaedia,"  Ar*  Correlation  of  Forces, 
Gyroscope,  etc.;  Tyndall's  "Crystalline  and  Molecular  Forces,"  in 
Manchester  Scienco  Lectures,  '73-4  ;  Crane's  "  Ball  Paradox,"  in  Pop- 
ular Science  Monthly,  Vol.  X,  p,  725. 


III. 

J3.TTR ACTION. 


"The  smallest  dust  which  floats  upon  the  wind 
Bears  this  strong  impress  of  the  Eternal  mind : 
In  mystery  round  it  subtle  forces  roll, 
And  gravitation  binds  and  guides  the  whole.n 

11  Attraction,  as  gravitation,  is  the  muscle  and  tendon  of  the  universe,  by 
which  its  mass  is  held  together  and  its  huge  limbs  are  wielded.  As  cohesion 
and  adhesion,  it  determines  the  multitude  of  physical  features  of  its  different 
parts.  As  chemical  or  interatomic  action,  it  is  the  final  source  to 
we  trace  all  material  changes" — ARNOTT* 


ANAL  YSIS. 


'  —ATTRACTIVE  AND  REPELLENT  FORCES. 


fc' 
O 

^H 

O 


1.  COHESION. 


II.  ATTRACTION 

OF 
GRAVITATION. 


2.  ADHESION.  •< 


f  1.  Definition  of  Cohesion. 

2.  Three  States  of  Matter. 

3.  Cohesion  acts  at  Insensible  Distances. 

4.  Liquids  tend  to  form  Spheres. 

5.  Solids  tend  to  form  Crystals. 

6.  Annealing  and  Tempering. 

7.  Rupert's  Drop. 

f  1.  Definition  and  Illustration  of  Adhesion. 
( (1.)  In  water. 

2.  Capillary  Attraction,  4  (2.)  In  mercury. 

1(3.)  Illustrations 

3.  Solution. 

4  Diffusion  of  Liquids. 

5.  Diffusion  of  Gases. 

6.  Osmose  of  Liquids. 
L  7.  Osmose  of  Gases. 

1.  Law  of  Gravitation. 

2.  Illustrations  of  Gravity. 

8.  Three  Laws  of  Weight. 

(1.)  Laws  of  falling  bodies 
(2.)  Equations     of    faUii'O 

bodies. 

(3.)  To  find  depth  of  well. 
(4.)  Bodies  thrown  upward. 

(1.)  Three  states  of  equilib- 
rium. 

(2.)  To  find  centre  of  gravity. 
(3.)  General  principles. 
(4.)  Physiological  facts. 

(1.)  Three  laws. 
(2.)  Centre  of  oscillation. 
(3.)  To  find  centre  of  oscilla- 
tion. 

(4.)  As  time-keeper. 
(5.)  Other  use*. 


4.  Falling 
Bodies. 


5.  Centre  of 
Gravity. 


The  Pen- 
dulum. 


I.     MOLECULAR     FORCES. 

Attractive  and  Repellent  Forces. — If  we  take  a 
piece  of  iron  and  attempt  to  pull  it  to  pieces,  we  find  that 
there  is  a  force  which  holds  the  molecules  together  and 
resists  our  efforts.  If  we  try  to  compress  the  metal,  we  find 
that  there  is  a  force  which  holds  the  molecules  apart  and 
resists  our  efforts  as  before.  If,  however,  we  apply  heat,  the 
iron  expands  and  finally  melts.  So,  also,  if  we  heat  a  bit  of 
ice,  the  attractive  force  is  gradually  overpowered,  the  solid 
becomes  a  liquid,  and  at  last  the  repellent  force  predomi-. 
nates  and  the  liquid  passes  off  in  vapor.  In  turn,  we  can 
cool  the  vapor,  and  convert  it  back  successively  into  water 
and  ice.  We  thus  see  that  there  are  two  opposing  forces 
which  reside  in  the  molecules — an  attractive  and  a  repellent 
force,  and  that  the  latter  is  heat.  There  are  three  kinds  of 
the  former,  cohesion,  adhesion,  and  chemical  affinity.  * 

1.    COHESION. 

1.  Cohesion  is  that  force  which  holds  together  molecules 
of  the  same  kind. 

2.  Three  States  of  Matter. — Matter  occurs  in  three 
states — solid,  liquid,  and  gaseous.     These  depend   on  the 
relation  of  the  attractive  and  repellent  forces,  cohesion  and 
heat.     If  they  are  nearly  balanced,  the  body  is  liquid;  if 
the  attractive  force  prevail,  it  is  solid ;  if  the  repellent,  it  is 
gaseous.    Many  substances  may  be  made  to  take  the  three 
states  successively.     Thus,  by  the  addition  of  heat,  ice  may 
be  converted  into  water,  and  thence  into  vapor ;    or  vice 

*  Chemical  affinity  produces  chemical  changes,  and  its  consideration  belongs  to 
Chemistry.  It  binds  together  atoms  of  different  kinds,  and  produces  a  compound 
unlike  the  original  elements. 


44  ATTRACTION. 

versa,  by  the  subtraction  of  heat.  Most  solids  pass  easily  to 
the  liquid  form,  others  go  directly  from  the  solid  to  the 
gaseous  state. 

3.  Cohesion  Acts  at  Insensible  Distances. — Take 
two  bullets,  and  having  flattened  and  cleaned  one  side  of 
each,  press  them  together  with  a  twisting  motion.     They 
will  cohere  when  the  molecules  are  crowded  into  apparent 
contact.  * — If  two  globules  of  mercury  be  brought  near  each 
other,  at  the  instant  they  seem  to  touch  they  will  suddenly 
coalesce. — Two  freshly-cut  surfaces  of  rubber,  when  warmed 
and  pressed  together,  will  cohere  as  if  they  formed  one  piece. 
— The  process   of    welding  illustrates  this  principle.      A 
wrought-iron  tool  being  broken,  we  wish  to  mend  it.     So 
we  bring  the  iron  to  a  white  heat  at  the  ends  which  we  in- 
tend to   unite.     This  partly  overcomes   the   attraction   of 
cohesion,   and  the   molecules   will   move   easily  upon   one 
another.     Laying  now  the  two  heated  ends  upon  each  other, 
we  pound  them  until  the  molecules  are  brought  near  enough 
for  cohesion  to  grasp  them. 

4.  Liquids  Tend  to  Form  Spheres. — Mix  a  glass  of 
water  and  alcohol  in  such  proportion  that  a  drop  of  sweet- 
oil  will  fall  half-way  to  the  bottom.     It  will  there  form  a 
perfect  sphere.     The  same  tendency  is  seen  in  dew-drops, 
rain-drops,  globules  of  quicksilver,  and  in  the  manufacture 
of  shot.     (Chemistry,  p.  174.)     The  reason  is  that  the  force 
of  cohesion  acts  toward  the  centre  of  the  drop.     In  a  spher- 
ical body,  every  portion  of  the  surface  is  equally  distant 
from  the  centre  ;   and  when  that  form  is  assumed,  every 
molecule  011  the  outside  is  equally  attracted,  and  an  equi- 
librium is  established. 

5.  Solids   Tend  to  Form   Crystals. — When  a  liquiu 
becomes  a  solid,  the  general  tendency  is  to  assume  a  sym- 

*  Surfaces  may  appear  to  the  eye  to  be  in  contact  when  they  are  not  actually  BO. 
Newton  found,  during  eome  experiments  on  light  (p.  168),  that  a  convex  lens  or  a 
watch-glass  laid  on  a  flat  glass  does  not  touch  it,  and  cannot  he  made  to  do  so,  even 
by  a  force  of  many  pounds. 


COHESIOK. 


45 


metrical  form.  The  attraction  of  cohesion  strives  to  arrange 
the  molecules  in  an  orderly  manner.  Each  kind  of  matter 
has  its  peculiar  shape  and  angle,  by  which  its  crystals  may 
be  recognized.  *  When  different  substances  are  contained  in 
the  same  solution,  they  separate  on  crystallization,  and  each 
molecule  goes  to  its  own.  The ,  exquisite  beauty  of  these 
crystalline  forms  is  seen  in  snowflakes  and  the  frostwork 
traced  on  a  cold  morning  upon  the  windows  or  the  stone- 
flagging.  A  beam  of  light  passed  through  a  block  of  ice 
reveals  these  crystals  as  a  mass  of  star-like  flowers  (Fig.l9).f 


FIG.  19. 


Melted  iron  rapidly  cooled  in  a  mould  has  not  time  to 
arrange  its  crystals.  If,  however,  the  iron  be  afterward 
violently  jarred,  as  when  used  for  cannon,  rail-cars,  etc.,  the 


*  Epsom  salt  crystallizes  in  four-sided  prism?,  common  salt  in  cubes,  and  alum  in 
octahedra.  We  can  illustrate  the  formation  of  the  last  by  adding  alum  to  hot  water 
until  no  more  will  dissolve.  Then  suspend  strings  across  the  dish  and  set  it  away  to 
cool.  Beautiful  octahedral  crystals  will  collect  on  the  threads  and  sides  of  the  vessel. 
The  slower  the  process,  the  larger  the  crystals.— God  delights  in  order  as  in  beauty. 
Down  in  the  dark  recesses  of  the  earth  He  has  fashioned,  by  the  slow  processes  of 
His  laws,  the  rarest  gems— amethysts,  rubies,  and  diamonds.  There  are  mountain 
masses  transparent  as  glass,  caves  hung  with,  stalactites,  and  crevices  rich  with  gold 
and  silver,  and  lined  with  quartz. 

t  It  is  noticeable  that,  as  the  crystals  melt,  at  the  centre  of  each  liquid  flower  is  a 
vacuum,  showing  that  there  is  rot  enough  water  formed  to  £11  the  space  occupied  by 
the  crystal,  and  that  the  solid  contracts  as  it  passes  into  a  fluid  (p.  202).  This  ex- 
periment is  easily  tried.  The  ice  must  be  cut  parallel  to  the  plane  of  its  freezing  and 
be  not  over  half  an  inch  thick.  A  common  oil-lamp  will  furnish  the  light. 


4  ATTRACTION. 

molecules  take  on  the  crystalline  form  and  the  metal  becomes 
brittle.  * 

6.  Annealing  and  Tempering. — If  a  piece  of  Drought- 
iron  be  heated  and  then  plunged  into  water,  it  becomes 
hard  and  brittle.     If,  on  the  contrary,  it  be  heated  and 
cooled  slowly,  it  is  made  tough  and  flexible.     Strangely 
enough,  the  same  process  which  hardens  iron  softens  copper. 
Steel  is  tempered  by  heating  white-hot,  then  cooling  quickly, 
and  afterward  re-heating  and  cooling  slowly.     The  higher 
the  temperature  of  the  second  heating,  the  softer  the  steel. 
(Chemistry,  p.  152.) 

7.  The  Rupert's  Drop  is  a  tear  of  melted  glass  dropped 

into  water,  and   cooled   quickly.     As   there  is 

PIG  20        no^    ^me*  ^or  the  particles    to    assume    their 

-1  natural  position,   they  exert   a  violent   strain 

I    \        upon  one  another ;  and  if  the  tail  of  the  drop 

be  nipped  off,  the  tension  will  cause  the  mass 

yv  to  fly  into  powder  with  a  sharp  explosion.     All 

^^       glassware,  when  first  made,  is  brittle,  but  it  is 

annealed  by  being  drawn  slowly  through  a  long 

oven,  highly  heated  at  one  end,  but  quite  cool 

at  the  other.     During  this  passage,  the  molecules  of  glass 

have  time  to  arrange  themselves  in  a  stable  position,  f 

PRACTICAL  QUESTIONS.— 1.  Why  can  we  not  weld  a  piece  of  copper  to  one  of 
iron?  2.  Why  is  a  bar  of  iron  stronger  than  one  of  wood?  3.  Why  is  a  piece  of  iron, 
when  perfectly  welded,  stronger  than  before  it  was  broken  ?  4.  Why  do  drops  of  dif- 
ferent liquids  vary  in  size  ?  5.  When  you  drop  medicine,  why  will  the  last  few  drops 
contained  in  the  bottle  be  of  a  larger  size  than  the  others  ?  6.  Why  are  the  drops 
larger  if  you  drop  them  slowly  ?  7.  Why  is  a  tube  stronger  than  a  rod  of  the  same 
weight?  8.  Why,  if  you  melt  scraps  of  lead,  will  they  form  a  solid  mass  when 
cooled  ?  9.  In  what  liquids  is  the  force  of  cohesion  greatest  ?  10.  Name  some  solids 
which  volatilize  without  melting.  11.  Why  can  glass  be  welded  ? 

*  On  examining  such  a  piece  of  iron,  which  can  easily  be  procured  at  a  car  or 
machine-shop,  we  can  see  in  a  fresh  fracture  the  smooth,  shiny  face  of  the  crystals. 

t  "  The  restoration  of  cohesion  is  beautifully  seen  in  the  gilding  of  china.  A  figure 
is  drawn  upon  the  china  with  a  mixture  of  oxide  of  gold  and  an  essential  oil.  The 
article  is  then  heated,  whereby  the  essential  oil  and  the  oxygen  of  the  gold  are 
expelled,  and  a  red-brown  pattern  remains.  This  consists  of  pure  gold  in  a  finely- 
divided  state,  without  lustre.  By  rubbing  with  a  hard  burnisher,  the  particles  of  gold 
cohere  and  reflect  the  rich  yellow  color  of  the  polished  metal." 


MOLECULAR    FORCES. 


FIQ.  21. 


2.    ADHESION. 

1.  Adhesion  is  the  force  which  holds  together  molecules 
of  different  kinds.     Ex.  :  Two  pieces  of  wood  are  fastened 
together  with  glue,  two  pieces  of  china  with  cement,  two 
bricks  with  mortar,  two  sheets  of  paper  with  mucilage,  and 
two  pieces  of  tin  with  solder.     Syrup  and  coal-oil  are  puri- 
fied by  filtering  through  animal  char- 
coal.    Bubbles  can  be  blown  from  soap- 
suds, because  the  soap  by  its  adhesive 

force    holds    together  the  particles  of 
water. 

2.  Capillary  Attraction  (capillus, 
hair)  is  a  variety  of  adhesion  between 
solids  and  liquids.     It  may  be  seen  when 
two  panes  of  glass  are  placed  as  shown 
in  Fig.  21,  but  is  exhibited  most  strik- 
ingly in  very  fine  tubes,  whence  the  name.* 

If  we  insert  a  glass  tube  in  ivater,  the  liquid  will  rise  in 
it.  The  finer  the  bore  of  the  tube,  the 
higher  the  ascent.  The  same  in  less  de- 
gree exists  between  glass  and  alcohol.  If 
we  insert  a  glass  tube  in  mercury,  the 
capillary  attraction  will  be  reversed,  and 
the  height  of  the  liquid  will  be  lower  than 
the  general  level. 

All  parts  of  a  liquid  are  mobile,  but  the 
surface  is  in  a  state  of  strain  because  the 
downward  pull  of  the  molecules  beneath  is 
not  balanced.     There  is  a  tough  film  which 
is  strong  enough  to  confine  the  body  of  the  liquid.     It  is 
this  "surface  tension "f  that  gives  roundness  to  the  pearly 

*  These  tubes  may  be  drawn  by  the  pupil  to  any  length  and  size  from  French 
glass  tubing  in  the  heat  of  an  alcohol  lamp. 

t  For  discussion  of  surface  tension,  see  Maxwell's  Theory  of  Heat,  p.  280  ;  Descha- 
nel's  Natural  Philosophy,  p.  130 ;  Popular  Science  Monthly,  Vol.  IX.,  p.  575 ;  Ameri- 
can Journal  of  Science,  Dec.  1882 ;  Pickering's  Physical  Manipulation,  Vol.  I.,  p.  102. 


FIG.  22. 


48  ATTRACTION. 

dew-drop,  strength  to  the  brilliant  soap-bubble,  and  holds 
up  the  slender  column  of  liquid  in  a  capillary  tube. 

FIG.  23.  ILLUSTRATIONS. — The  wick  of  a  lamp  or 

candle  is  a  bundle  of  capillary  tubes,  which 
elevate  the  oil  or  melted  fat  and  feed  the 
flame. — If  the  end  of  a  towel  be  dipped  in  a 
basin  of  water,  the  whole  towel  will  soon  be 
wet  by  capillary  action  through  the  pores  of 
the  cloth. — Blotting-paper  takes  up  ink  by 
capillarity. — Water  in  the  saucer  of  a  flower- 
pot is  elevated  through  the  pores  of  the  earth 
to  the  plant.* — Ropes  absorb  water  by  capil- 
lary action,  swell,  and  shrink  often  to 
breaking,  f 

3.  Solution. — Sugar  will  dissolve  in  water,  because  the 
adhesion  between  the  two  substances  is  stronger  than  the 
cohesion  of  the  sugar,  J  As  heat  weakens  cohesion,  it 

*  In  the  same  way,  water  is  drawn  to  the  surface  of  the  ground  to  furnish  vege- 
tation with  the  materials  of  growth.  Even  in  the  winter,  when  the  surface  is  frozen, 
the  water  still  finds  its  way  upward,  and  freezes  into  ice,  which  in  the  spring  pro- 
duces mud,  although  there  may  have  been  little  rain  or  snow.  Stirring  the  ground 
causes  it  better  to  endure  drought,  because  the  size  of  the  capillary  pores  is  increased, 
thus  preventing  the  water  from  being  carried  to  the  surface  and  evaporated. 

t  It  is  1586.  The  Egyptian  obelisk,  weighing  a  million  pounds,  is  to  be  raised 
in  the  square  of  St.  Peter's,  Rome.  Pope  Sixtus  V  proclaims  that  no  one  shall 
utter  a  word  aloud  until  the  engineer  announces  that  all  danger  is  passed.  As 
the  majestic  column  ascends,  all  eyes  watch  it  with  wonder  and  awe.  Slowly  it 
rises,  inch  by  inch,  foot  by  foot,  until  the  task  is  almost  completed,  when  the  strain 
becomes  too  great.  The  huge  ropes  yield  and  slip.  The  workmen  are  dismayed,  and 
fly  wildly  to  escape  the  impending  mass  of  stone.  Suddenly  a  voice  breaks  the 
silence.  "Wet  the  ropes,"  rings  out  clear-toned  as  a  trumpet.  The  crowd  look. 
There,  on  a  high  post,  standing  on  tiptoe,  his  eyes  glittering  with  the  intensity  of 
excitement,  is  one  of  the  eight  hundred  workmen,  a  sailor  named  Bresca  di  S.  Remo. 
His  voice  and  appearance  startle  every  one ;  but  his  words  inspire.  He  is  obeyed. 
The  ropes  swell  and  bite  into  the  stone.  The  column  ascends  again,  and  in  a  moment 
more  stands  securely  on  its  pedestal.  The  daring  sailor  is  not  only  forgiven,  but 
his  descendants  to  this  day  enjoy  the  reward  of  providing  the  palm-branches  used  on 
Palm  Sunday  at  St.  Peter's. 

$  This  contest  between  adhesion  and  cohesion  is  seen  when  we  let  fall  on  water  a 
drop  of  oil.  Adhesion  tends  to  draw  the  oil  to  the  liquid,  BO  as  to  mix  thoroughly, 
and  cohesion  to  prevent  this.  The  extent  to  which  the  drop  will  spread  will  depend 
on  the  relation  of  the  two  attractions,  and  vary  for  every  substance.  Thus  each  oil 
nas  its  own  COHESION  FIGURE,  which  enables  the  chemist  readily  to  detect  differences 
and  mixtures.  Experiments  :  Dissolve  a  little  salt  in  a  glass  of  water,  and  touch 


ADHESION.  49 

hastens  solution,  so  that  a  substance  generally  dissolves 
more  rapidly  in  hot  water  than  in  cold.  In  like  manner, 
pulverizing  a  solid  aids  solution.  Liquids  also  absorb  gases 
by  adhesion.  Thus  water  contains  air,  which  renders  it 
pleasant  to  the  taste.  As  pressure  and  cold  weaken  the 
repellent  force,  they  favor  the  adhesion  between  the  mole- 
cules of  a  gas  and  water.  Soda-water  receives 
its  effervescence  and  pungent  taste  from  car- 
bonic-acid gas,  which,  being  absorbed  under 
great  pressure,  escapes  in  sparkling  bubbles 
when  the  pressure  is  removed. 


4.  Diffusion  of  Liquids. — Let  a  jar  be 
partly  filled  with  water  colored  by  blue  litmus. 
Then,  by  a  funnel-tube,  pour  clear  water  con- 
taining oil  of  vitriol  to  the  bottom,  beneath 
the  colored  water.  At  first,  the  two  will  be 
distinctly  defined,  "but  in  a  few  days  they  will 
mix,  as  will  be  seen  by  the  change  of  color  from  blue  to  red. 
A  drop  of  oil  of  vitriol  may  thus  be  distributed  through  a 
quart  of  water.  Most  liquids  will  mingle  when 
brought  in  contact.*  If,  however,  there  is  no  ad- 
hesion between  their  molecules,  they  will  not  mix, 
and  will  separate  even  after  having  been  thoroughly 
shaken  together. 

5.  Diffusion  of  G-ases. — Hydrogen  gas  is  only 
-^  as  heavy  as  common  air.  Yet,  if  two  bottles  be 
arranged  as  in  Fig.  25,  the  lower  one  filled  with  the 
heavy  gas,  and  the  upper  with  the  lighter,  the  gases 
will  soon  be  uniformly  mixed,  f 

the  surface  of  the  liquid  with  a  pen  full  of  ink.  The  characteristic  figures  will 
quickly  appear.— Dissolve  in  water  a  pinch  of  salt  and  a  lump  of  loaf-sugar.  Touch 
the  surface  with  lunar  caustic.  The  figure  of  nitrate  of  silver  will  be  seen. 

*  A  story  is  told  of  some  negroes  in  the  West  Indies  who  supplied  themselves 
with  liquor  by  inverting  the  neck  of  a  bottle  of  water  in  the  bung-hole  of  a  cask  of 
rum.  The  water  sank  into  the  barrel,  while  the  rum  rose  to  take  its  place. 

t  This  phenomenon  is  explained  by  the  theory  that  the  molecules  of  all  bodies  are 
in  rapid  motion.  As  the  worlds  in  space  are  clustered  in  mighty  systems,  the  mem- 


50 


6.  Osmose  of  Liquids. — When  two  liquids  are  sepa- 
rated by  a  thin  porous  substance,  the  interchange  is  modified 
in  a  curious  manner,  according  to  the  nature  of  the  liquid 
and  the  substance  used.  At  the  end  of  a  glass  tube  (Fig.  26) 
fasten  a  bladder  of  alcohol.  Fill  the  jar  with  water,  and 
mark  tL*  height  to  which  the  alcohol  ascends  in  the  tube. 


PIG.  26. 


FIG.  27. 


The  column  will  soon  begin  to  rise  slowly.  On  examination, 
we  shall  see  that  the  alcohol  is  passing  out  through  the  pores 
of  the  bladder  and  mixing  with  the  water,  while  the  water 

bers  of  each  revolving  about  one  another  in  inconceivably  vast  orbits,  so  each  body 
is  a  miniature  system,  its  molecules  moving  in  inconceivably  minute  paths.  In  a  gas, 
the  molecular  velocity  is  enormous.  The  particles  of  ammonia  gas,  for  example, 
are  flying  to  and  fro  at  the  rate  of  twenty  miles  per  minute.  "  Could  we,  by  any 
means,"  says  Prof.  Cooke,  "  turn  in  one  direction  the  actual  motion  of  the  molecules 
of  what  we  call  still  air,  it  would  become  at  once  a  wind  blowing  seventeen  miles 
per  minute,  and  exert  a  destructive  power  compared  with  which  the  most  violent 
tornado  is  feeble." — Invert  a  bottle  over  a  lighted  candle,  and  the  oxygen  of  the  en- 
closed air  being  soon  consumed  the  flame  goes  out.  Instead  of  the  bottle,  use  a 
foolscap-paper  cone.  There  will  be  an  interchange  of  gases  through  the  pores  of  the 
paper  and  the  light  will  burn  freely. 


AUHES10K.  51 

is  coming  in  more  rapidly.     In  most  other  cases  of  osmose, 
the  flow  is  toward  the  denser  liquid. 

The  chemist  uses  a  method  of  separating  substances  in 
solution,  termed  dialysis,  that  is  based  on  their  unequal 
diffusibility. 

7.  Osmose  of  Gases. — Fit  a  porous  cup  used  in  Grove's 
Battery  (p.  234)  with  a  cork  and  glass  tube,  as  in  Fig.  27. 
Fasten  the  tube  so  that  it  will  dip  beneath  the  water  in  the 
glass.  Then  invert  over  the  cup  a  jar  of  'hydrogen.  The 
gas  will  pass  through  the  pores  of  the  earthenware  and  down 
the  tube  so  rapidly,  as  almost  instantly  to  bubble  up  through 
the  water.* 

PRACTICAL  QUESTIONS.— 1.  Why  does  cloth  shrink  when  wet?  ,2.  Why  do 
sailors  at  a  boat-race  wet  the  sails  ?  3.  Why  is  writing-paper  sized  ?  4.  Why  does 
paint  prevent  wood  from  shrinking?  5.  What  is  the  shape  of  the  surface  of  a  glass- 
full  of  water  ?  Of  mercury  ?  6.  Why  can  we  not  perfectly  dry  a  towel  by  wringing? 
7.  Why  will  not  water  run  through  a  fine  sieve  when  the  wires  are  greased  ?  8.  Why 
will  camphor  dissolve  in  alcohol,  and  not  in  water  ?  9.  Why  will  mercury  rise  in  zinc 
tubes  as  water  will  in  glass  tubes  ?  10.  Why  is  it  so  difficult  to  lift  a  board  out  of 
water  ?  11.  Why  will  ink  spilled  on  the  edge  of  a  book  extend  farther  inside  than  if 
spilled  on  the  side  of  the  leaves  ?  12.  If  you  should  happen  to  spill  some  ink  on  the 
edge  of  your  book,  ought  you  to  press  the  leaves  together  ?  13.  Why  can  you  not 
mix  water  and  oil  ?  14.  What  is  the  object  of  the  spout  on  a  pitcher  ?  Ans.  The 
water  would  run  down  the  side  of  the  pitcher  by  the  force  of  adhesion,  but  the  spout 
throws  it  into  the  hands  of  gravitation  before  adhesion  can  catch  it.  15.  Why  will 
water  wet  your  hand,  while  mercury  will  not?  16.  Why  is  a  pail  or  tub  liable  to  fall 
to  pieces  if  not  filled  with  water  or  kept  in  a  damp  place  ?  17.  Name  instances  where 
the  attraction  of  adhesion  is  stronger  than  that  of  cohesion.  18.  Why  does  the  water 
in  Fig.  22  stand  higher  inside  of  the  tube  than  next  the  glass  on  the  outside  ?  19.  Why 
will  clothes-lines  tighten  and  sometimes  break  during  a  shower  ?  20.  Show  that  the 
law  of  the  diffusion  of  gases  aids  in  preserving  the  purity  of  the  atmosphere.  21.  In 
casting  large  cannon,  the  gun  is  cooled  by  a  stream  of  cold  water.  Why?  22.  Why 
does  paint  adhere  to  wood  ?  Chalk  to  the  blackboard  ?  23.  Why  does  a  towel  dry 
one's  face  after  washing?  24.  Why  will  a  greased  needle  float  on  water?  25.  Why 
is  the  point  of  a  pen  slit  ?  26.  Why  is  a  thin  layer  of  glue  stronger  than  a  thick  one  ? 

*  Rose  balloons  lose  their  buoyancy,  because  the  hydrogen  escapes  through  the 
pores  of  the  rubber.  If  they  were  filled  with  air  and  placed  in  a  jar  of  hydrogen,  that 
gas  would  creep  in  so  rapidly  as  to  burst  them.— The  quicker  flow  from  the  thinner 
to  the  thicker  fluid  is  termed  endosmose,  and  the  opposite,  slower  current,  exosmose, 
— In  performing  the  experiment  shown  in  Fig.  27,  coal-gas  may  be  used. 


II.     ATTRACTION     OF     GRAVITATION. 


FIG.  28. 


We  have  spoken  of  the  attraction  existing  between  the 
molecules  of  bodies  at  minute  distances.  We  now  notice  an 
attraction  which  acts  at  all  distances. 

1.  Law  of  Gravitation. — Hold  a  stone  in  the  hand, 
and  you  feel  a  power  constantly  drawing  it  to  the  ground. 
We  call  this  familiar  phenomenon  weight.  It  is  really  the 
attraction  of  the  earth  pulling-ihe  stone  back  to  itself — an 
instance  of  a  general  law,  one  operation  of  an  ever-active 
force.  For  every  particle  of  matter  in 
the  universe  *  attracts  every  other  parti' 
cle,  the  force  exerted  between  any  tivo 
particles  being  directly  proportional  to 
the  product  of  their  masses,  and  inversely 
as  the  square  of  their  distance  apart. 

Gravitation  is  the  general  term  for 
the  attraction  that  exists  between  all 
bodies  in  the  universe.  Gravity  is  the 
earth's  attraction  for  terrestrial  bodies  ; 
it  tends  to  draw  them  toward  the  centre 
of  the  earth.  Weight  is  the  measure  of 
the  force  of  gravity.  When  we  say  that 
a  body  weighs  10  Ibs.,  we  mean  that 
the  earth  attracts  it  that  amount. 

2.  Illustrations  of  Gravity. — A 
stone  falls  to  the  ground  because  the 

*  The  force  of  gravitation  resides  in  every  particle  of  matter,  and  hence  it  is  not 
confined  to  our  own  world.  By  its  action  the  heavenly  bodies  are  bound  to  one 
another,  and  thus  kept  in  their  orbits.  It  may  help  us  to  conceive  how  the  earth  is 
supported,  if  we  imagine  the  sun  letting  down  a  huge  cable,  and  every  star  in  the 
heavens  a  tiny  thread,  to  hold  our  globe  in  its  place,  while  it  in  turn  sends  back  a 
cable  to  the  sun  and  a  thread  to  every  one  of  the  stars.  So  we  are  bound  to  them  and 
they  to  us.  Thus  the  worlds  throughout  space  are  linked  together  by  these  cords  of 
mutual  attraction,  which,  interweaving  in  every  direction,  make  the  universe  a  unit. 


ATTRACTION    OF    GBAVITATION.  53 

earth  attracts  it ;  but  in  turn  the  stone  attracts  the  earth. 
Each  moves  to  meet  the  other,  but  the  stone  passes  through 
as  much  greater  distance  than  the  earth  as  its  mass  is  less. 
The  mass  of  the  earth  is  so  gpeat  that  its  motion  is  imper- 
ceptible.— A  plumb-line  hanging-near  a  mountain  is  attracted 
from  the  vertical.  In  Fig.  28,  AB  represents  the  ordinary 
position  of  the  line,  while  AC  indicates  the  attractive  power 
(exaggerated)  of  the  mountain.* 

3.  Laws  of  "Weight. — I.  The  iveight  of  a  body  at  the 
centre  of  the  earth  is  nothing,  because  the  attraction  there  is 
equal  in  every  direction. 

TL^Wie  weight  of  a  body  above  the  surface  of  the  earth 
decreases  as  the  square  of  the  distance  from  the  centre  of  the 
earth  increases.^ 

III.  TJie  weight  of  a  body  varies  on  different  portions  of  the 
surface  of  the  earth.  \  It  will  be  least  at  the  equator,  because 
(1),  on  account  of  the  bulging  form  of  our  globe,  a  body  is 
pushed  out  from  the  mass  of  the  earth,  and  so  removed  from 
the  centre  of  attraction ;  and  (2),  the  centrifugal  force  is 
the  strongest.  It  will  be  the  greatest  at  the  poles,  because 
(1),  on  account  of  the  flattening  of  the  earth,  a  body  is 


*  Maskelyne,  in  1774,  found  the  attraction  of  Mount  Schehallien  to  be  12".  By 
comparing  this  force  with  that  of  the  earth,  the  specific  gravity  of  the  mountain  being 
known,  the  specific  gravity  of  the  earth  was  estimated  to  be  5  times  that  of  water. 
Later  investigations  make  it  5.67. 

t  A  body  at  the  surface  of  the  earth  (4000  miles  from  the  centre)  weiglfls  100  Ibs. 
What  would  be  its  weight  1000  miles  above  the  surface  (5000  miles  from  the  centre)  ? 
SOLUTION.  (5000  mi.)a  :  (4000  mi.)a  ::  100  Ibs.  :  x  =  64  Ibs.  Or,  its  weight  would 

4000  3 
decrease  in  the  ratio  of  — —  =  |g.    Hence  it  would  weigh  |g  x  100  Ibs.  =  64  Ibs.— The 

weight  of  a  body  below  the  surface  of  the  earth  is  commonly  said  to  decrease  directly 
as  the  distance  from  the  centre  decreases.  Thus,  1000  miles  below  the  surface,  a 
body  would  lose  £  its  weight.  In  fact,  however,  the  density  of  the  earth  increases  so 
much  toward  the  centre,  that  for  "T7o  of  the  distance  the  force  of  gravity  actually  be 
comes  stronger  than  on  the  surface." 

$  In  these  statements  concerning  weight,  a  spring-balance  is  supposed  to  be  em 
ployed. '  With  a  pair  of  scales,  the  weights  used  would  become  heavier  or  lighter  in 
the  same  proportion  as  the  body  to  be  weighed.  If  a  spring-scale  be  graduated  to 
indicate  correctly  afa  medium  latitude,  it  would  show  too  little  at  the  equator,  and 
too  much  at  the  poles.  In  other  words,  a  pound  weighed  by  such  a  spring-scale  at 
the  equator  would  contain  a  greater  mass  of  matter  than  one  weighed  at  the  polea  by 
about  jjj  part, 


54 


ATTBACTIOH. 


Fio.  89. 


brought  nearer  its  mass  and  the  centre  of  attraction ;  and 
(2),  there  is  no  centrifugal  force  at  those  points. 

4.  Falling  Bodies. — Since  the  attraction  of  the  earth  is 
toward  its  centre,  bodies  falling  freely  move  in  a  direct  line 
toward  that  point.     This  line  is  called 
a  vertical  or  plumb-line.  * 

(1.)  LAWS  or  FALLING  BODIES. — 
I.  Under  the  influence  of  gravity  alone, 
all  bodies  fall  with  equal  rapidity. 
This^is  well  illustrated  by  the  "  guinea- 
and-feather  experiment."  Let  a  coin 
and  a  feather  be  placed  in  a  tube,  and 
the  air  exhausted.  Quickly  invert  the 
tube,  and  the  two  will  fall  in  nearly 
the  same  time.  Let  in  the  air  again, 
and  the  feather  will  flutter  down  long 
after  the  coin  has  reached  the  bottom,  f 
Hence  we  conclude  that  in  a  vacuum 
all  bodies  descend  with  equal  velocity, 
and  that  the  resistance  of  the  air  is  the 
cause  of  the  variation  we  see  between 
the  falling  of  light  and  of  heavy 
bodies. 

II.  In  the  first  second  a  body  gains 
a  velocity  of  32  feet  and  falls  \§feet.\ 
—This  has  been  proved  by  careful  experiments.     Notice 
that  16  feet,  the  distance  passed  through  the  first  second,  is 
the  mean  between  0,  the  velocity  at  the  beginning,  and  32, 
the  velocity  at  the  close. 

*  Prom  plumbum,  lead,  because  a  lead  weight  is  used  by  mechanics  in  finding  it. 
All  plumb-lines  point  very  nearly  toward  the  centre  of  the  earth. 

t  The  same  fact  may  be  noticed  in  the  case  of  a  sheet  of  paper.  When  spread  opt, 
it  merely  flutters  to  the  ground  ;  but  when  rolled  in  a  compact  mass,  it  falls  like  lead. 
In  this  case  we  have  not  increased  the  force  of  attraction,  but  we.have  diminished  the 
resistance  of  the  air. 

t  More  exactly,  at  the  latitude  of  New  York  a  body  will  fall  in  a  vacuum  16.08  feet 
J)jo  first  second,  and  gain  a  velocity  of  32.16  feet.  16  ft.  =4,9  m.  32  ft.  =9.8  m. =980  cm. 


GRAVITATION.  66 

HI.  At  the  end  of  any  given  second,  the  velocity  is  16  feet 
multiplied  by  tivice  the  number  of  the  second ;  and  the  dis- 
tance passed  through  during  that  second  is  16  feet  multiplied 
by  twice  the  number  of  the  second  minus  one.  In  other  words, 
the  velocities  are  as  the  corresponding  even  numbers,  2,  4, 

6,  8,  etc.,  and  the  distances  as  the  odd  *  numbers,  1,  3,  5, 

7,  etc. 

The  body  commences  the  second  second  with  a  velocity  of 
32  feet,  and  as  gravity  is  a  constant  force,  gains  32  feet 
during  the  second,  making  64  feet  —  4  x  16  feet.  It  com- 
mences the  third  second  with  a  velocity  of  64  feet,  and  gains 
32  feet,  making  96  feet  =  6  x  16  feet.  The  mean  between 
32  feet,  the  velocity  at  the  beginning  of  the  second  second, 
and  64  feet,  the  velocity  at  the  close,  is  48  feet  =  3  x  16  ft. 
The  mean  between  64  feet,  the  velocity  at  the  beginning  of 
the  third  second,  and  96  feet,  the  velocity  at  the  close,  is 
80  feet  =  5  x  16  feet. 

IV.  In  any  number  of  seconds  a  body  falls  16  feet  multi- 
plied by  the  square  of  the  number  of  seconds. 

AVe  have  just  seen  that  a  body  falls  16  feet  the  first  second 
and  48  feet  the  next.  Hence  in  two  seconds  it  falls 
16  feet  -f-  48  feet  =  64  feet  =  22  x  16  feet.  In  three  sec- 
onds it  falls  16  +  48-^  80  feet  =  144  feet  =  32  x  16  feet. 

(2.)  EQUATIONS  OF  FALLING  BODIES. — If  we  represent 
the  velocity  of  a  falling  body  by  v,  the  distance  in  any  second 
by  5,  the  total  distance  by  d,  and  the  time  by  t,  the  follow- 
ing equations  can  be  derived  from  the  foregoing  laws  : 

v  =  82*.  .(1).        d  =  16£2. . (2).        -»2  =  64d.  .(3).        s  =  16  (2t  -  1).  .(4). 

If  g  represent  the  constant  force  of  gravity,  a  velocity  of 
32  feet  per  second,  we  have, 


.  .(6).  «  =  ^gd  .....  (8).  a  =  %ff(2t  -  1).  .  .  .(10). 


*  It  will  aid  the  memory,  If  we  associate  d  in  "distance"  and  ''odd,"  and  win 
"velocity"  and  "•  even."—  To  find  the  odd  number  corresponding  to  any  second, 
double  the  number  of  the  second  and  subtract  one.  See  Explanatory  Tables,  p.  273. 


56  ATTRACTION. 

(3.)  To  FIND  THE  DEPTH  CF  A  WELL. — Let  a  stone  fall 
into  it,  and,  with  a  watch  or  by  the  beat  of  the  pulse,  count 
the  seconds  that  elapse  before  you  hear  it  strike  the  bottom. 
Square  the  number  of  seconds,  multiply  16  feet  by  the  result, 
and  the  product  is  the  depth.* 

(4.)  WHEN  A  BODY  is  THROWN  UPWARD,  the  same 
principles  apply,  it  losing  through  gravity  32  feet  in  velocity 
each  second.  The  velocity  necessary  to  elevate  it  to  a  cer- 
tain point  must  be  what  it  would  acquire  in  falling  that 
distance,  f  It  will  rise  just  as  high  in  a  given  time  as  it 
would  fall  in  the  same  time.  If  a  ball  be  thrown  vertically 
into  the  air,  it  will  be  as  long  in  falling  as  in  rising.  In 
theory,  it  will  strike  the  earth  with  the  same  velocity  with 
which  it  was  thrown ;  in  practice,  however,  it  loses  some  of 
its  velocity  in  rising  and  an  equal  amount  in  falling,  owing 
to  the  resistance  of  the  air. 

5.  The  Centre  of  Gravity  is  that  point  on  which,  if 
supported,  a  body  will  balance  itself.  The 
tine  of  direction  is  a  vertical  drawn  from 
the  centre  of  gravity  ;  it  is  the  line  along 
which  the  centre  of  gravity  would  pass,  if 
the  body  should  fall.  When  a  body  is  at 
rest,  the  forces  which  act  on  every  mole- 
cule in  it  are  said  to  balance  one  another, 
or  to  be  in  equilibrium. 

(1.)  THREE  STATES  OF  EQUILIBRIUM. — 
1st.  A  body  is  in  stable  equilibrium  when 
the  centre  of  gravity  is  below  the  point  of 
support,  or  when  any  movement  tends  to 
raise  the  centre  of  gravity.  In  Fig.  30, 
the  image  has  the  centre  of  gravity  lowered 
below  the  point  of  support  by  means  of 

*  A  little  time  is  required  for  the  sound  to  come  to  the  ear,  but  this  is  eo  slight 
that  it  may  be  neglected. 

t  If  a  body  be  thrown  upward  with  a  velocity  of  128  feet,  by  applying  equation  (9) 

t  =  - ,  we  find  that  it  will  rise  for  4  seconds. 
ff 


GRAVITATION. 


57 


Fia.  31. 


lead  balls.     Remove  these,  and  it  immediately  falls,  but  with 

them  it  is  in  stable  equilib- 
rium.    Any  movement  of  the 

toy  shown  in  Fig.  31  tends  to 

raise  the  centre  of  gravity, 

and  it  returns  quickly  to  a 

state  of  rest. — A  needle  may 

be  balanced  on  its  point  by  a 

cork    and    two    jack-knives 

(Fig.    32),    which  lower  its 

centre  of  gravity. 
2d.  A  body  is  said  to  be 

in  unstable  equilibrium  when 

the  centre  of  gravity  is  above 

the  point  of  support,  or  when 

any  movement  tends  to  lower 

the  centre  of  gravity.  If  we  take  the  cork  as  arranged 
with  the  knives  in  Fig.  32,  and  invert  it, 
we  shall  have  difficulty  in  balancing 
the  needle ;  and,  if  we  succeed,  it  will 
readily  topple  off,  as  the  least  motion 
tends  to  lower  the  centre  of  gravity. 

3d.  A  body  is  said  to  be  in  indifferent 
equilibrium  when  the  centre  of  gravity 
is  at  the  point  of 
support,  or  when 

any  movement  tends  neither  to  ele- 

rate  nor  lower  the  centre  of  gravity. 

A  ball  of  uniform  density  on  a  level 

surface  will  rest  in  any  position,  be- 
cause the  centre  of  gravity  moves  in 

a  line  parallel  to  the  floor. 
(2.)   THE   CENTRE   OF    GRAVITY 

MAY  BE  FOUND  either  by  balancing 

the  body,  or  by  suspending  it  from 

two  corners,  successively,  as  in  Fig. 

33.     By  a    plumb-line    obtain    the 


PIG.  33. 


58  ATTRACTION. 

line  of  direction  AE ;  then  hang  the  slate  from  another  cor- 
ner, and  mark  the  line  of  direction  BD.  The  point  0, 
where  the  two  lines  cross,  is  the  centre  of  gravity. 

(3.)  GENERAL  PRINCIPLES. — (a.)  The  centre  of  gravity 
tends  to  seek  the  lowest  point. 

(b.)  A  body  will  not  tip  over  while  the  line  of  direction 
falls  within  the  base,  but  will  as  soon  as  it  falls  without.* 

(c.)  In  general,  narrowness  of  base  combined  with  height 
of  centre  of  gravity,  tends  to  instability ;  f  breadth  of  base 
and  lowness  of  centre  of  gravity,  produce  stability. 

(4.)  PHYSIOLOGICAL  FACTS. — Our  feet  and  the  space  be- 
tween them  form  the  base  on  which  we  stand.  By  turning 
our  toes  outward,  we  increase  its  breadth.  When  we  stand 
on  one  foot,  we  bend  over  so  as  to  bring  the  line  of  direction 
within  this  narrower  base.  When  we  walk,  we  incline  to 
the  right  and  the  left  alternately.  When  we  carry  a  pail  of 
water,  we  balance  it  by  leaning  in  the  opposite  direction. 
When  we  walk  up  hill  we  lean  forward,  and  in  going  down 
hill  we  incline  backward,  in  unconscious  obedience  to  the 
laws  of  gravity.  We  bend  forward  when  we  wish  to  rise 
from  a  chair,  in  order  to  bring  the  centre  of  gravity  over  our 
feet,  our  muscles  not  having  sufficient  strength  to  raise  our 
bodies  without  this  aid.  When  we  walk,  we  lean  forward, 
so  as  to  bring  the  centre  of  gravity  as  far  in  front  as  possi- 

*  The  Leaning  Tower  of  Pisa,  in  Italy,  beautifully  illustrates  this  principle  (see 
Frontispiece).  It  is  about  188  feet  high,  and  its  top  leans  15  feet,  yet  the  line  of 
direction  falls  so  far  within  the  base  that  it  is  perfectly  stable,  having  stood  for  seven 
centuries.  The  feeling  experienced  by  a  person  who  for  the  first  time  looks  down 
from  the  lower  side  of  the  top  of  this  apparently  impending  structure  is  startling 
indeed. 

t  This  is  shown  by  the  difficulty  in  learning  to  walk  upon  stilts.  The  art  of  bal- 
ancing one's  self  may,  however,  be  acquired  by  practice,  as  is  seen  in  the  Landes  of 
southwestern  France.  During  a  portion  of  the  year  these  sandy  plains  are  half- 
covered  with  water,  and  in  the  remainder  are  still  very  bad  walking.  The  natives 
accordingly  double  the  length  of  their  legs  by  stilts.  Mounted  on  these  wooden  poles, 
which  are  put  on  and  off  as  regularly  as  the  other  parts  of  their  dress,  they  appear  to 
strangers  as  a  new  and  extraordinary  race,  marching  with  steps  of  six  feet  in  length, 
and  with  the  speed  of  a  trotting-horse.  While  watching  their  flocks,  they  support 
themselves  by  a  third  staff  behind,  and  then  with  their  rough  sheep-skin  cloaks  and 
caps,  like  thatched  roofs,  seem  to  be  little  watch-towers,  or  singular  lofty  tripods, 
scattered  over  the  country.— (Arnott.) 


GRAVITATION. 


59 


ble.  Thus,  walking  is  a  process  of  falling. 
When  we  run,  we  lean  further  forward,  and 
so  fall  faster.  (Phys.,  p.  49.) 

6.  The  Pendulum  consists  of  a  weight 
so  suspended  as  to  swing  freely.  Its  move- 
ments to  and  fro  are  termed  vibrations  or 
oscillations.  The  path  through  which  it 
passes  is  called  the  arc.  The  extent  to  which 
it  goes  in  either  direction  from  the  lowest 

O 

point  is  styled  its  amplitude.  Vibrations 
performed  in  equal  times  are  termed  i-socti- 
ro-nous  (isos,  equal ;  chronos,  time). 

(1.)  THEEE  LAWS. — I.  In  the  same  pen- 
dulum, all  vibrations  of  small  amplitude  are 
isochronous.     If  we  let  one  of  the  balls  rep-  = 
resented  fti  Fig.  34  swing  through  a  short 

arc,   and  then 

through  a  longer  one,  on 
counting  the  number  of  oscil- 
lations per  minute,  we  shall 
find  them  very  uniform. 

II.  Tlie  times  of  the  vibra- 
tions of  different  pendulums 
are  proportional  to  the  square 
roots  of  their  respective  lengths.^ 
Ex.  :  A  pendulum  \  the  length 
of  another,  will  vibrate  three 
times  as  fast.  *  Conversely,  the 
lengths  of  different  pendulums 
are  proportional  to  the  squares 
of  their  times  of  vibration. 


PIG.  35. 


*  A  pendulum  which  vibrates  seconds 
must  be  four  times  as  long  as  one  which 
vibrates  half-seconds.  The  apparatus  rep- 
resented in  Figs.  34  and  35  can  be  made  by 
any  carpenter  or  ingenious  pupil,  and  will 
serve  excellently  to  illustrate  the  three  laws 
of  the  pendulum. 


60 


ATTRACTION. 


Fl6  s6-  III.  The  time  of  the  vibration  of  the 

same  pendulum  ivill  vary  at  different 
places,  since  it  decreases  as  the  square 
root  of  the  force  of  gravity  increases. 
At  the  equator  a  pendulum  vibrates 
most  slowly,  and  at  the  poles  most  rap- 
idly. The  length  of  a  seconds-pendulum 
at  New  York  is  about  39^  inches. 

(2.)  CENTRE  OF  OSCILLATION. — The 
upper  part  of  a  pendulum  tends  to  move 
faster  than  the  lower  part,  and  so  has- 
tens the  speed.  The  lower  part  of  a 
pendulum  tends  to  move  slower  than 
the  upper  part,  and  so  retards  the  speed. 
Between  these  extremes  is  a  point  which 
is  neither  quickened  nor  impeded  by  the 
rest,  but  moves  in  the  same  time  that  it 
would  if  it  were  a  particle  swinging  by 
an  imaginary  line.  This  point  is  called 
the  centre  of  oscillation.  It  lies  a  little 
below  the  centre  of  gravity.*  In  Fig.  35 
is  shown  an  apparatus  containing  pen- 
dulums of  different  shapes,  but  of  the 
same  length.  If  they  are  started  to- 
gether, they  will  immediately  diverge, 
no  two  vibrating  in  the  same  time.  As 
pendulums,  they  are  not  of  the  same 
length. 

(3.)  THE  CENTRE  OF  OSCILLATION 
is  FOUND  BY  TRIAL. — Huygens  dis- 
covered that  the  point  of  suspension  and 
the  centre  of  oscillation  are  interchange- 
able. If,  therefore,  a  pendulum  be  inverted,  and  a  point 
found  at  which  it  will  vibrate  in  the  same  time  as  before, 

*  This  determines  the  real  length  of  a  pendulum,  which  is  the  distance  from  the 
point  of  support  to  the  centre  of  oscillation.  The  imaginary  pendulum  above 
described  is  known  in  Physics  as  the  Simple  Pendulum.— 39.1  inches  =  933.3  mm. 


GBAVITATION. 


61 


Fie.  37. 


this  is  the  former  centre  of  oscillation  ;  while  the  old  point 
of  suspension  becomes  the  new  centre  of  oscillation.* 

(4.)  THE  PENDULUM  AS  A  TIME-KEEPER. — The  friction 
at  the  point  of  suspension,  and  the  resistance  of  the  air, 
soon  destroy  the  motion  of  the  pendulum.  The  clock  is  a 
machine  for  keeping  up  the  vibration  of  the  pendulum,  and 
counting  its  beats.  In  Fig.  36,  R  is  the  scape-wheel  driven 
by  the  force  of  the  clock-weight  or  spring,  and  mn  the  escape- 
ment, moved  by  the  forked  arm  AB,  so  that 
only  one  cog  of  the  wheel  can  pass  at  each 
double  vibration  of  the  pendulum.  Thus 
the  oscillations  are  counted  by  the  cogs  on 
the  wheel,  while  the  friction  and  the  resist- 
ance of  the  air  are  overcome  by  the  action  of 
the  weight  or  spring. f  As  "heat  expands 
and  cold  contracts,"  a  pendulum  lengthens 
in  summer  and  shortens  in  winter.  A  clock, 
therefore,  tends  to  lose  time  in  summer 
and  gain  in  winter.  To  regulate  a  clock,  we 
raise  or  lower  the  pendulum-bob,  L,  by  the 
nut  v. 

The  gridiron  pendulum  consists  of  brass 
and  steel  rods,  so  connected  that  the  brass, 
h,  Tc,  will  lengthen  upward,  and  the  steel  «, 
£,  c,  d,  downward,  and  thus  the  centre  of 
oscillation  remain  unchanged.  The  mercu- 
rial pendulum  contains  a  cup  of  mercury 
which  expands  upward  while  the  pendulum- 
rod  expands  downward. 

(5.)  OTHER  USES  OF  THE  PEHDULUM. — 
(a.)  Since  the  time  of  vibration  of  a  pendu- 


*  The  centre  of  oscillation  is  the  same  as  the  centre  of  percussion.  The  latter  is 
the  point  where  we  must  strike  a  suspended  body,  if  we  wish  it  to  revolve  about  its 
axis  without  any  strain.  If  we  do  not  hit  a  ball  on  the  bat's  centre  of  percussion,  our 
hands  "  sting  "  with  the  jar.  (See  note,  p.  260.) 

t  The  action  of  &  clock  is  clearly  seen  by  procuring  the  works  of  an  old  clock  and 
watching  the  mcvczLccts  of  ike  various  parts. 


ATTRACTION. 


FIG.  38. 


lum  indicates  the  force  of  gravity,  and  the  force  of  gravity 
decreases  as  the  square  of  the  distance  from  the  centre  of 
the  earth  increases,  we  may  thus  find  the  semi-diameter  of 
the  earth  at  various  places,  and  ascertain  the  figure  of  our 

globe.  (b.)  Knowing  the 
force  of  gravity  at  any 
point,  the  velocity  of  a  fall- 
ing body  can  be  determined. 
(c.)  The  pendulum  may  be 
used  as  a  standard  of  meas- 
ures, (d.)  Foucault devised 
a  method  of  showing  the 
revolution  of  the  earth  on 
its  axis,  founded  upon  the 
fact  that  the  pendulum  vi- 
brates constantly  in  one 

plane.*  (e.)  By  observing  the  difference  in  the  length  of  a 
seconds-pendulum  at  the  top  of  a  mountain  and  at  the  level 
of  the  sea,  the  density  of  the  earth  may  be  estimated. 

PRACTICAL  QUESTIONS.  — 1.  When  an  apple  falls  to  the  ground,  how  much 
does  the  earth  rise  to  meet  it?  2.  What  causes  the  sawdust  on  a  mill-pond  to  collect 
in  large  masses?  3.  Will  a  body  weigh  more  in  a  valley  than  on  a  mountain  ?  4.  Will 
a  pound  weight  fall  more  slowly  than  a  two-pound  weight  ?  5.  How  deep  is  a  well  if 
it  takes  three  seconds  for  a  stone  to  fall  to  the  bottom  ?  13.  Is  the  centre  of  gravity 
always  within  a  body— as,  for  example,  a  pair  of  tongs  ?  7.  If  two  bodies,  weighing 
respectively  2  and  4  Ibs. ,  be  connected  by  a  rod  2  feet  long,  where  is  the  centre  of 
gravity  ?  8.  In  a  ball  of  equal  density  throughout,  where  is  the  centre  of  gravity  ? 
9.  Why  does  a  ball  roll  down  hill  ?  10.  Why  is  it  easier  to  roll  a  round  body  than  a 
square  one  ?  11.  Why  is  it  easier  to  tip  over  a  load  of  hay  than  one  of  stone  ? 
12.  Why  is  a  pyramid  such  a  stable  structure  ?  13.  Where  is  the  centre  of  gravity 
of  a  hollow  ball?  14.  Why  does  a  rope-walkbr  carry  a  heavy  balanchig-pole ? 
15.  What  would  become  of  a  ball  if  dropped  into  a  hole  bored  through  the  centre  of 
the  earth  ?  16.  Would  a  clock  lose  or  gain  time  }f  carried  to  the  top  of  a  mountain  ? 
If  carried  to  the  North  Pole  ?  17.  In  the  winter,  would  you  raise  or  lower  the  pendu- 
him-bob  of  your  clock?  18.  Why  is  the  pendulum-bob  often  made  flat?  19.  What 

*  A  pendulum  220  feet  in  length  was  suspended  from  the  dome  of  the  Pantheon  in 
Paris.  The  lower  end  of  the  pendulum  traced  its  vibrations  north  and  south  upon  a 
table  beneath,  sprinkled  with  fine  sand.  These  paths  did  not  coincide,  but  at  each 
return  to  the  outside,  the  pendulum  marked  a  point  to  the  right,  At  the  poles  of  the 
earth,  the  pendulum,  constantly  vibrating  in  the  same  vertical  plane,  would  perform 
a  complete  revolution  in  24  hoars,  making  thus  a  kind  of  clock.  At  the  equator  it 
would  not  change  east  or  west,  as  the  plane  of  vibration  would  go  forward  with  the 
diurnal  revolution  of  the  earth.  The  shifting  of  the  plane  would  increase  as  the  pen- 
dnlom  was  carried  north  or  south  from  the  equator. 


SUMMARY.  63 

"beats  off"  the  time  In  a  watch?  20.  What  should  be  the  length  of  a  pendulum  to 
vibrate  minutes  at  the  latitude  of  New  York  ?  Solution.  (1  sec.) a :  (60  sec.)8 : :  89 1  in. 
:  x=2.2+  miles.  21.  What  should  be  the  length  of  the  above  to  vibrate  half-seconds  ? 
Quarter-seconds  ?  Hours  ?  22.  What  is  the  proportionate  time  of  vibration  of  two 
pendulums,  respectively  16  and  64  inches  long?  23.  Why,  when  you  are  standing 
erect  against  a  wall,  and  a  piece  of  money  is  placed  between  your  feet,  can  you  not 
stoop  forward  and  pick  it  up  ?  .  24.  If  a  tower  were  198  feet  high,  with  what  velocity 
would  a  stone,  dropped  from  the  summit,  strike  the  ground  ?  25.  A  body  falls  hi 
5  seconds ;  with  what  velocity  does  it  strike  the  ground  ?  26.  How  far  will  a  body 
fall  in  10  seconds?  With  what  velocity  will  it  strike  the  ground?  27.  A  body  is 
thrown  upward  with  a  velocity  of  192  feet  the  first  second ;  to  what  height  will  it 
rise?  28.  A  ball  is  shot  upward  with  a  velocity  of  256  feet;  to  what  height  will  it 
rise  ?  How  long  will  it  continue  to  ascend  ?  29.  Why  do  not  drops  of  water,  falling 
from  the  clouds,  strike  with  a  force  proportional  to  the  laws  of  falling  bodies  ? 
Am.  Because  they  are  so  small  that  the  resistance  of  the  air  nearly  destroys  their 
velocity.  If  it  were  not  for  this  wise  provision,  a  shower  of  rain-drops  would  be  as 
fatal  as  one  of  Minie  bullets.  30.  Are  any  two  plumb-lines  parallel  ?  31.  A  stone  let 
fall  from  a  bridge  strikes  the  water  in  3  seconds.  What  is  the  height  ?  32.  A  stone 
falls  from  a  church-steeple  in  4  seconds.  What  is  the  height  of  the  steeple  ?  83.  How 
far  would  a  body  fall  in  the  first  second  at  a  distance  of  12,000  miles  above  the  earth's 
surface  ?  34.  A  body  at  the  surface  of  the  earth  weighs  100  tens ;  what  would  be  its 
weight  1,000  miles  above  ?  35.  A  boy  wishing  to  find  the  height  of  a  steeple,  lets  fly 
an  arrow  that  just  reaches  the  top  and  then  falls  to  the  ground.  It  is  in  the  air  6  sec- 
onds. Required  the  height.  36.  A  cat  let  fall  from  a  balloon  reaches  the  grouni  in 
10  seconds.  Required  the  distance.  37.  In  what  time  will  a  pendulum  40  feet  long 
make  a  vibration  ?  38^  Two  meteoric  bodies  in  space  are  12  miles  apart.  They 
weigh  respectively  100  and  200  Ibs.  If  they  should  fall  together  by  their  mutual 
attraction,  what  portion  of  the  distance  would  be  passed  over  by  each  body?  39.  If 
a  body  weighs  2,000  Ibs.  upon  the  surface  of  the  earth,  what  would  it  weigh  2,000 
miles  above  ?  500  miles  above  ?  40.  At  what  distance  above  the  earth  will  a  body 
fall,  the  first  second,  21 1  inches?  41.  How  far  will  a  body  fall  in  8  seconds?  In  the 
8th  second  ?  In  10  seconds  ?  In  the  30th  second  ?"'\42.  How  long  would  it  take  for  a 
pendulum  one  mile  in  length  to  make  a  vibration  ?N^3.  How  long  must  a  pen- 
dulum be  to  vibrate  three  times  in  5  seconds  ?  44^(jVill  a  pendulum  made  of 
lead  vibrate  faster  than  one  of  the  same  length  made  of  feathers  ?^  Which  would 
come  to  rest  sooner  ?  Why  ?  45.  What  would  be  ,,he  time  of  Vibration  of  a 
pendulum  64  metres  long?  46.  A  ball  is  dropped  from  a  height  of  64  feet. 
At  the  same  moment  a  second  ball  is  thrown  upward  with  sufficient  velocity 
to  reach  the  same  point.  Where  will  the  two  balls  pass  each  other  ?  47.  Two  bodies 
are  successively  dropped  from  the  same  point  with  an  interval  of  J  of  a  second. 
When  will  the  distance  between  them  be  one  metre  ?  48.  Explain  the  following 
fact :  A  straight  stick  loaded  with  lead  at  one  end,  can  be  more  easily  balanced  ver- 
tically on  the  finger  when  the  loaded  end  is  upward  than  when  it  is  downward.  49. 
What  effect  would  the  fall  of  a  heavy  body  to  the  earth  have  upon  the  motion  of  the 
earth  in  its  orbit?  (In  answering  this  question,  imagine  the  body  to  fall  in  various 
directions  toward  the  earth,  as  opposed  to  the  motion  of  the  earth,  in  the  same 
direction  with  the  earth's  motion,  etc.)  50.  If  a  body  weighing  a  pound  on  the  earth 
were  carried  to  the  sun  it  would  weigh  about  27  pounds.  How  much  would  it  then 
attract  the  sun  ?  51.  Why  does  watery  vapor  float  and  rain  fall  ?  52.  If  a  body 
weighs  10  kilos,  on  the  surface  of  the  earth,  what  would  it  weigh  1,000  kilometres 
above  (the  earth's  radius  being  6.366  km.)  ?  53.  A  body  is  thrown  vertically  upward 
with  a  velocity  of  100  metres  ;  how  long  before  it  will  return  to  its  original  position  ? 
54.  Required  the  time  needed  for  a  body  to  fall  a  distance  of  2,000  metres.  55.  If  two 
bodies,  weighing  respectively  1  kilo,  and  1  demi-kilo.,  are  connected  by  a  rod  90 
centimetres  long,  where  is  the  centre  of  gravity  ? 


64  ATTRACTION. 


SUM  MARY. 

There  are  certain  forces  residing  in  molecules  and  acting  only  at 
insensible  distances,  which  are  known  as  the  Molecular  Forces.  The 
one  which  ties  together  molecules  of  the  same  kind  is  styled  cohesion. 
The  relation  between  this  force  and  that  of  heat  determines  whether  a 
body  is  solid,  liquid,  or  gaseous.  Under  the  action  of  cohesion,  liquids 
tend  to  form  spheres  ;  and  solids,  crystals.  The  processes  of  welding 
and  tempering,  and  the  annealing  of  iron  and  glass,  illustrate  curious 
modifications  of  the  cohesive  force.  Molecules  of  different  kinds  are 
held  together  by  adhesion.  Its  action  is  seen  in  the  use  of  cement, 
paste,  etc.,  in  the  solution  of  solids,  in  capillarity,  diffusion  of  gases, 
and  osmose. 

Gravitation,  though  weak,*  compared  with  cohesion,  acts  uni- 
versally. Its  force  is  directly  as  the  product  of  the  attracting  and 
attracted  masses,  and  inversely  as  the  square  of  their  distance  apart. 
Gravity  makes  a  stone  fall  to  the  ground.  The  earth  and  a  kilogram 
of  iron  in  mid-air  attract  each  other  equally,  but  the  former  is  so  much 
heavier  that  they  move  toward  each  other  with  unequal  velocity,  and 
the  motion  of  the  earth  is  imperceptible.  Weight  is  the  resisted 
attraction  of  the  earth.  At  the  centre  of  the  earth  the  weight  of  a 
body  would  be  nothing ;  at  the  poles  it  would  be  greatest,  and  at  the 
equator  least.  Increase  of  distance  above  or  far  below  the  surface  of 
the  earth  will  diminish  weight.  Were  the  resistance  of  the  air 
removed,  all  bodies  would  fall  with  equal  rapidity.  The  first  second 
a  body  falls  16  feet  (4.9  metres),  and  gains  a  velocity  of  32  feet 
(9.8  metres).  In  general,  the  velocity  of  a  falling  body  is  16  feet, 
multiplied  by  the  even  number  corresponding  to  the  second,  and  the 
distance  16  feet  multiplied  by  the  odd  number.  The  centre  of  gravity 
is  the  point  about  which  the  weights  of  all  the  particles  composing  a 
body  will  balance  one  another,  i.  e.,  be  in  equilibrium.  There  are  three 
states  of  equilibrium — stable,  unstable,  and  indifferent — according  as 
the  point  of  support  in  a  body  is  above,  below,  or  at  the  centre  of  grav- 
ity. As  the  centre  of  gravity  tends  to  seek  the  lowest  point,  its  position 
determines  the  stability  of  a  body.  The  centre  of  gravity  may  be 
found  by  trial  or  with  a  plumb-line.  A  body  suspended  so  as  to  swing 
freely  is  a  pendulum.  The  time  of  a  pendulum's  vibration  is  inde- 
pendent of  its  material,  proportional  to  the  square  root  of  its  length  and 

*  As  the  attraction  of  gravitation  acts  so  commonly  upon  great  masses  of  matter, 
we  are  apt  to  consider  it  a  tremendous  force.  We,  however,  readily  detect  its  rela- 
tive feebleness  when  we  compare  the  weight  of  bodies  with  their  tenacity.  Ex. : 
Think  how  much  easier  it  is  to  lift  an  iron  wire  against  gravity  than  to  pull  it  to 
pieces  against  cohesion. 


HISTORICAL  SKETCH.  65 

variable  according  to  the  latitude.     The  pendulum  is  our  time-keeper 
and  useful  in  many  scientific  investigations. 

We  are  so  accustomed  to  see  all  the  objects  around  us  possess 
weight,  that  we  can  hardly  conceive  of  a  body  deprived  of  a  property 
which  we  are  apt  to  consider  as  an  essential  attribute  of  matter. 
Nothing  is  more  natural,  apparently,  than  the  falling  of  a  stone  to  the 
ground.  "Yet,"  says  D'Alembert,  "it  is  not  without  reason  that 
philosophers  are  astonished  to  see  a  stone  fall,  and  those  who  laugh 
at  their  astonishment  would  soon  share  it  themselves,  if  they  would 
reflect  on  the  subject."  Gravity  is  constantly  at  work  about  us,  at  one 
moment  producing  equilibrium  or  rest,  and  at  another,  motion.  When 
it  seems  to  be  destroyed,  it  is  only  counterbalanced  for  a  time,  and 
remains,  apparently,  as  indestructible  as  matter  itself.  The  stability 
and  the  incessant  changes  of  nature  are  alike  due  to  its  action.  Not 
only  do  rivers  flow,  snows  fall,  tides  rise,  and  mountains  stand  in 
obedience  to  gravitation,  but  smoke  ascends  and  clouds  float  through 
the  combined  influence  of  heat  and  weight. 


HISTORICAL    SKETCH. 

The  latter  part  of  the  sixteenth  century  witnessed  the  establish- 
ment of  the  principles  of  falling  bodies.  Galileo,  while  sitting  in  the 
cathedral  at  Pisa  (see  Frontispiece)  and  watching  the  swinging  of  an 
immense  chandelier  which  hung  from  its  lofty  ceiling,  noticed  that  its 
vibrations  were  isochronous.  This  was  the  germ-thought  of  the  pen- 
dulum and  the  clock.  Up  to  his  time  it  had  been  taught  that  a  4-lb. 
weight  would  fall  twice  as  fast  as  a  2-lb.  one.  He  proved  the  fallacy 
of  this  view  by  dropping  from  ths  Leaning  Tower  of  Pisa  balls  of  dif- 
ferent metals — gold,  copper,  and  lead.  They  all  reached  the  ground  at 
nearly  the  same  moment.  The  slight  variation  he  correctly  accounted 
for  by  the  resistance  of  the  air,  which  was  not  the  same  for  all. 

Newton,  as  the  story  runs,  was  sitting  in  his  garden  one  day,  and 
noticed  the  fall  of  an  apple.  Reflecting  upon  the  force  which  drew  it 
to  the  ground,  the  thought  struck  his  mind  that  perhaps  the  same  force 
acted  upon  the  heavenly  bodies.  The  moon,  for  example,  revolves 
about  the  earth  in  a  fixed  orbit.  Might  it  not  be  the  attraction  of  the 
earth  which  causes  the  moon  to  move  in  this  curved  path?  To  test 
this,  he  calculated  how  far  the  moon  bends  from  a  straight  line,  i.  e. , 
falls  toward  the  earth  every  second.  Knowing  the  distance  a  body 
falls  in  a  second  at  the  surface  of  the  earth,  he  endeavored  to  see  how 
far  it  would  fall  at  the  distance  of  the  moon.  For  years  he  toiled  over 


66  ATTRACTION. 

this  problem,  but  an  erroneous  estimate  of  the  earth's  diameter  then 
accepted  by  physicists  prevented  his  obtaining  a  correct  result.  Finally, 
a  more  accurate  measurement  having  been  made,  he  inserted  this  in 
his  calculations.  Finding  the  result  was  likely  to  verify  his  conjecture, 
his  hand  faltered  with  the  excitement,  and  he  was  forced  to  ask  a  friend 
to  complete  the  task.  The  truth  was  reached  at  last,  and  the  grand 
law  of  gravitation  discovered  (1682). 

The  sun-dial  was  doubtless  the  earliest  device  for  keeping  time. 
The  clepsydra  was  afterward  employed.  This  consisted  of  a  vessel  con- 
taining water,  which  slowly  escaped  into  a  dish  below,  in  which  was  a 
float  that  by  its  height  indicated  the  lapse  of  time.  King  Alfred  used 
candles  of  a  uniform  size,  six  of  which  lasted  a  day.  The  first  clock 
erected  in  England,  about  1288,  was  considered  of  so  much  importance 
that  a  high  official  was  appointed  to  take  charge  of  it.  The  clocks  of 
the  middle  ages  were  extremely  elaborate.  They  indicated  the  motions 
of  the  heavenly  bodies ;  birds  came  out  and  sang  songs,  cocks  crowed, 
and  trumpeters  blew  their  horns  ;  chimes  of  bells  were  sounded,  and 
processions  of  dignitaries  and  military  offieers,  in  fantastic  dress, 
marched  in  front  of  the  dial  and  gravely  announced  th3  time  of  day. 
Watches  were  made  at  Nuremberg  in  the  fifteenth  century.  They 
were  styled  Nuremberg  eggs.  Many  were  as  small  as  the  watches  of 
the  present  day,  while  others  were  as  large  as  a  dessert-plate.  They 
had  no  minute  or  second  hand,  and  required  winding  twice  per  day. 

On  Attraction,  as  well  as  on  subsequent  topics  treated  in  this 
book,  consult  Guillemin's  "  Forces  of  Nature  "  ;  Atkinson's  Ganot's 
Physics  ;  Arnott's  "  Elements  of  Physics";  SnelPs  Olmstead's  Natural 
Philosophy  ;  Todhunter's  "  Philosophy  for  Beginners "  ;  Stewart's 
Elementary  Physics  ;  Silli  man's  Physics  ;  Everett's  "  Text-book  of 
Physics";  Young's  "Lectures  on  Natural  Philosophy";  Thomson 
and  Tait's  "Elements  of  Natural  Philosophy";  "Appleton's  Cyclo- 
paedia," Articles  on  Clocks  and  Watches,  Weights  and  Measures,  Gravi- 
tation, Mechanics,  etc. ;  Peck's  Ganot's  Natural  Philosophy  ;  Miller's 
Chemical  Physics,  Chap.  Ill,  on  Molecular  Force ;  Weinhold's  Experi- 
mental Physics;  Pickering's  "  Elementary  Physical  Manipulation"; 
Fourteen  Weeks  in  Astronomy,  Sections  on  Galileo  and  Newton,  pp. 
29-34. 

The  current  numbers  of  Harper's  Magazine ;  Scribner's  Magazine 
(The  World's  Work) ;  Popular  Science  Monthly ;  Boston  Journal  of 
Chemistry  ;  Scientific  American ;  Knowledge ;  and  Nature,  contain  the 
latest  phases  of  science. 


IV. 


Nature  is  a  reservoir  of  power.  Tremendous  forces  are  all  about  us, 
but  they  are  not  adapted  to  our  use.  We  need  to  remould  the  energy  to  Jit 
our  wants.  A  waterfall  cannot  grind  corn  nor  the  wind  draw  water. 
Yet  a  machine  will  gather  up  these  wasted  forces,  and  turn  a  grist-mill  or 
work  a  pump.  A  kettle  of  boiling  water  has  little  of  promise ;  but 
husband  its  energy  in  the  steam-engine,  and  it  will  weave  cloth,  forge  an 
anchor,  or  bear  our  burdens  along  the  iron  track. 

"  The  hero  in  the  fairy  tale  had  a  servant  who  could  eat  granite  rocks, 
another  who  could  hear  the  grass  grow,  and  a  third  who  could  run  a  hun- 
dred leagues  in  half  an  hour.  So  man  in  nature  is  surrounded  by  a  gang 
of  friendly  giants  who  can  accept  harder  stints  than  these.  There  is  no 
porter  like  gravitation,  who  will  bring  down  any  weight  you  cannot  carry, 
and  if  he  wants  aid,  knows  how  to  get  it  from  his  fellow-laborers.  Water 
sets  his  irresistible  shoulder  to  your  mill,  or  to  your  ship,  or  transports  vast 
boulders  of  rock,  neatly  packed  in  his  iceberg,  a  thousand  miles" 

—EMERSON 


CQ 

w 


<1 


ANALYSIS. 


r— THE  SIMPLE  MACHINES. 
—THE  LAW  OF   MECHANICS. 

1.  Definition. 

"(1.) 


1.  THE  LEVER. 


£  4.  Steelyard. 

[jj  L  5.  Compound  Lever. 

O 


2.  Three   Classes 

of  Levers 
™' 


(3.*)  Third  Class. 


3.  Law  of  Equilibrium. 


2.  THE    WHEEL    AND)*'  Definition  and  Illustration. 


w  AVI  p  •{  2.  Law  of  Equilibrium. 

*  [  3.  Wheelwork. 

ft 

O   1  3.  THE  INCLINED  PLANE,  j  1-  Definition  and  Illustration. 

j  2.  Law  of  Equilibrium. 

EH 
fc 
(^ 

P 

H 

w 


4.  THE  SCREW  J  *'  ^enni*ion  an^  Illustration. 

(  2.  Law  of  Equilibrium. 

5    THE  WEDGE  j  *'  ^ennition  an^  Illustration. 


i 


Law  of  Equilibrium. 


1.  Definition  and  Illustration. 

6    THE  PULLEY  J  ^'  ^xed  and  Movable  Pulleys, 

3.  Combinations  of  Pulleys. 

4.  Law  of  Equilibrium. 

7.  CUMULATIVE  CONTRIVANCES. 

8.  PERPETUAL  MOTION. 


ELEMENTS     OF     MACHINES. 

The  Simple  Machines  are  the  elements  to  which  all 
machinery  can  be  reduced.  The  watch  with  its  complex 
system  of  wheel-work,  and  the  engine  with  its  belts,  cranks 
and  pistons,  are  only  various  modifications  of  some  of  the 
six  elementary  forms — the  lever,  the  wheel  and  axle,  the 
inclined  plane,  the  screiv,  iho^wedge,  and  the  pulley.  * 

They  are  often  termed  the  Mechanical  Powers,  but  they 
do  not  produce  work  ;  they  are  only  methods  of  applying  it. 
Here  again  the  doctrine  of  the  Conservation  of  Energy 
holds  good.  The  work  done  by  the  power  is  always  equal 
to  the  resistance  overcome  in  the  weight. 

The  Law  of  Mechanics  is,  the  power  multiplied  ly  the 
distance  through  which  it  moves,  is  equal  to  the  weight  multi- 
plied by  the  distance  through  which  it  moves,  f  Ex. :  1  Ib.  of 
power  moving  through  10  feet  =  10  Ibs.  of  weight  moving 
through  one  foot,  or  vice  versa.  This  must  include  the  work 
of  moving  the  machine  as  well  as  the  weight  itself. 

1.  The  Lever  is  a  bar  turning  on  a  pivot.  The  force 
used  is  termed  the  poiver  (P),  the  object  to  be  lifted  the 
iveight  (W),  the  pivot  on  which  the  lever  turns  the  fulcrum 
(F),  and  the  parts  of  the  lever  each  side  of  the  fulcrum  the 
arms. 

THREE  CLASSES  OF  LEVERS. — In  the  three  kinds,  the 
fulcrum,  weight  and  power  are  each  respectively  between 
the  other  two,  as  may  be  seen  by  comparing  Figs.  39-41. 

*  These  six  may  be  still  further  reduced  to  two  —  the  lever  and  the  inclined 
plane. 

t  In  theory,  the  parts  of  a  machine  have  no  weight,  move  with  no  friction,  and 
meet  no  resistance  from  the  air.  In  practice,  these  influences  must  be  considered. 


70 


ELEMENTS  OF  MACHINES. 


Fie. 


I 


FIG.  40. 


Fie.  41. 


FIG.  42. 


TV' 

First  Class. — We  wish  to  lift  a  heavy  stone.  Accordingly 
we  put  one  end  of  a  handspike 
under  it,  and  resting  the  bar  on  a 
block  at  F,  bear  down  at  P. — A 
pump-handle  is  a  lever  of  the  first 
class.  The  hand  is  the  P,  the  water 
lifted  the  W,  and  the  pivot  the  F.— 
A  pair  of  scissors  is  a  double  lever  of 
the  same  class.  The  cloth  to  be 

cut  is  the  W,  the  hand  the  P,  and  the  rivet  the  F. 
Second    Class. — We    may    also 

raise  the  stone,  as  in  Fig.  43,  by  FIO.  43. 

resting  one  end  of  the  lever  on  the 

ground,  which  acts  as  a  fulcrum, 

and  lifting  up   on  the  bar. — An 

oar  is  a  lever  of  the  second  class. 

The  hand  is  the  P,  the  boat  the 

W,  and  the  water  the  F. 

Third  Class. — The  treadle  of  a  sewing-machine  is  a  lever 

of  the  third  class.     The  front  end  resting  on  the  ground  is 

the  F,  the  foot  is  the  P,  and  the  force  is  transmitted  by  a 

rod  to  the  W,  the  arm  above. — In  the  fishing-rod,  one  hand 

is  the  F,  the  other  the  P,  and  the  fish  the  W.  * 

LAW  OF  EQUILIBRIUM. — A  force  multiplied  by  its  per- 
pendicular distance  from  a  point  is  called  the  moment  or 
turning  effort  of  the  force  about  that  point  as  a  pivot.  In 
the  lever,  P  balances  W  when  the  moments  about  the  ful- 
crum are  equal.  Let  Pd  represent  power's  distance  from 

*  See  p.  xi.    Fresh  Facts  and  Theories. 


THE  LEVER. 


71 


the  F,  and  Wd  weight's  distance. 
the  law  of  mechanics, 


Substituting  these  terms 


P:W::We2:P<Z(2). 


The  P  is 


FIG.  44. 


In  the  first  and  second  classes,  as  ordinarily  used,  we  gain 
power  and  lose  time  ;  in  the  third  class  we  lose  power  and 
gain  time. 

The  STEELYARD  is  a  lever  of  the  first  class, 
at  E,  the  F  at  0,  and  the  W  at  D.     If  the  dis- 
tance from  the  pivot  of  the  hook  D  to  the 
pivot  of  the  hook  C  be  one  inch,  and  from  the 
pivot  of  the  hook  0   to  the  notch  where  E 
hangs  be  12  inches,  then  1  Ib.  at  E  will  balance 
12  Ibs.   at  W.      If   the 
steelyard    be     reversed    A 

(Fig.  45),  then  the  dis-    (j| -j^^^^^^^^^^ 

tance  of  the  F  from  the  A 

W  is  only  }  as  great,  and 

1  Ib.  at  E  will  balance 

48  Ibs.  at  D.     Two  sets  of  notches  on  opposite 

sides  of  the  bar  correspond  to  these    different 

positions. 

The  COMPOUND  LEVER  consists  of  several  levers 
so  connected  that  the  short  arm  of  the  first  acts 
on  the  long  arm  of  the  second,  and  so  on  to  the 
last  of  the  series.     If  the  distance  of  A  (Fig.  46) 
from  the  F  be  four  times  that  of  B,  a  P  of  5  Ibs. 
at  A  will  lift  a  W  of 
20  Ibs.    at  B.      If  the 
arms  of  the  second  lever 
are  of  the  same  com- 
parative length,  a  P  of 

20  Ibs.  at  C  will  balance  80  Ibs.  at  E.  In  the  third 
lever,  a  P  of  80  Ibs.  at  D.  will  lift  320  Ibs.  at  F. 
With  this  system  of  three  levers,  5  Ibs.  at  A 
will  accordingly  balance  320  Ibs.  at  F.  To  raise 


FIG.  45. 


ELEMENTS   OF  MACHINES. 


FIG.  47. 


Fl0-46-  the  W  1   foot,  however, 

the  P  must  move  64  feet. 
Thus  what  is  gained  in 
power  is  lost  in  time. 
There  is  no  creation  of 
force  by  the  use  of  the 
levers  ;  the  rather,  there  is  a  small  loss  because  of  friction. 

Hay  scales  are  constructed  upon  the  principle  of  the 
compound  lever.  The 
platform  rests  on  sev- 
eral levers,  as  shown  in 
Fig.  47.  These  are  con- 
nected with  a  side  lever, 
which  is  notched  to  in- 
dicate the  pounds,  etc. 
The  weight  is  moved 
along  this  lever  until  it 
balances  the  load  of  hay. 

2.  The  "Wheel  and  Axle  is  a  kind  of  perpetual  lever. 
As  both  arms  work  continuously,  we  are  not  obliged  to  prop 
up  the  W  and  readjust  the  lever.     In 
FIG.  48.  the  windlass  used  for  drawing  water 

from  a  well,  the  P  is  applied  at  the 
handle,  the  W  is  the  bucket,  and  the  F 
is  the  axis  of  the  windlass.  The  long 
arm  of  the  lever  is  the  length  of  the 
handle,  and 
the  short  arm 
is  the  semi- 
diameter  o  f 
FIG.  50.  the  axle. 

This  is 
shown  in 
a  cross- 
section 
(Fig.  48) 
where  0 


FIG.  49. 


THE    WHEEL    AND    AXLE.  73 

is  the  F,  0  A  the  long  arm,  and  0  B  the  short  arm. — In 
Fig.  49,  instead  of  turning  a  handle  we  take  hold  of  pins 
inserted  in  the  rim  of  the  wheel. — Fig.  50  represents  a 
capstan  used  on  vessels  for  lifting  the  anchor.  The  P  is 
applied  by  handspikes  inserted  in  the  axle. — Fig.  51  shows  a 
form  of  the  capstan  employed  in  moving  buildings,  in  which 
a  horse  furnishes  the  power. 

FIG.  51. 


LAW  OF  EQUILIBRIUM. — By  turning  the  handle  or  wheel 
around  once,  the  rope  will  be  wound  around  the  axle  and 
the  W  be  lifted  that  distance.  Applying  the  law  of 
mechanics,  P  x  the  circumference  of  the  wheel  =  W  x  cir- 
cumference of  the  axle ;  or,  as  circles  are  proportional  to 
their  radii, 

P  :  W  :  :  radius  of  the  axle :  radius  of  the  wheel.         (4) 

WHEELWORK  consists  of  a  series  of  Fle-  52- 

wheels  and  axles  which  act  upon  one 
another  on  the  principle  of  the  co*m- 
pound  lever.  The  cogs  on  the  cir- 
cumference of  the  wheel  are  termed 
teeth,  on  the  axle  leaves,  and  the  axle 
itself  is  called  a  pinion.  If  the  radius 
of  the  wheel  F  be  12  inches,  and 
that  of  the  pinion  2  inches,  then  a  P  of  1  Ib.  will  apply  a 


74 


ELEMENTS    OF    MACHINES. 


force  of  6  Ibs.  to  the  second  wheel  E.  If  the  radius  of  this 
be  12  inches,  then  the  second  wheel  will  apply  a  P  of  36  Ibs. 
to  the  third  wheel,  which,  acting  on  its  axle,  will  balance  a 
W  of  216  Ibs.* 

3.  The  Inclined  Plane. —If  we  wish  to  lift  a  heavy 
cask  into  a  wagon,  we  rest  one  end  of  a  plank  on  the  wagon- 
box  and  the  other  on  the  ground.  We  can  then  easily  roll 
the  cask  up  this  inclined  plane.  When  roads  are  to  be  made 

FIG.  53. 


FIG.  54. 


over  steep  hills,  they  are  sometimes  constructed  around  the 
hill,  like  the  thread  of  a  screw,  or  in  a  winding  manner  as 
shown  in  Fig.  53.  f 

LAW  OF  EQUILIBRIUM. — In 
Fig.  54  the  P  must  descend  a 
distance  CA  to  elevate  the  W 
to  the  height  BC.  Applying 
the  law  of  mechanics,  P  x 

*  The  W  will  pass  through  only  ^  the  distance  of  the  P.  We  thus  gain  power 
and  lose  speed.  If  we  wish  to  reverse  this  we  can  apply  the  P  to  the  axle,  and  so 
gam  speed.  This  is  the  plan  adopted  in  factories,  where  a  water-wheel  furnishes 
abundant  power,  and  spindles  or  other  swift  machines  are  to  be  turned  with  great 
rapidity. 

+  There  is  a  remarkable  ascent  of  this  kind  on  Mount  Royal,  Montreal. 


THE    INCLINED    PLANE.  75 

length  of  inclined  plane  =  W  x  height  of  inclined  plane  ; 
hence, 
P  :  W  : :  height  of  inclined  plane  :  length  of  inclined  plane.  *  (5.)  - 

If  a  road  ascend  1  foot  in  100  feet,  then  a  horse  drawing 
up  a  wagon  has  to  lift  only  -^  of  the  load,  besides  over- 
coming the  friction.  A  body  sliding  down  a  smooth  inclined 
plane  acquires  the  same  velocity  that  it  would  in  falling  the 
same  height  perpendicularly.  A  train  descending  a  grade 
of  1  foot  in  100  feet  tends  to  go  down  with  a  force  equal  to 
YJ-g-  of  its  weight,  f 

4.  The  Screw  consists  of  an  inclined  plane  wound 
around  a  cylinder,  the  former  being 
called  the  thread,  and  the  latter  the 
body.  It  works  in  a  nut  which  is  fitted 
with  reverse  threads  to  move  on  the 
thread  of  the  screw.  The  nut  may  turn 
on  the  screw,  or  the  screw  in  the  imt. 
The  P  may  be  applied  to  either,  by 
means  of  a  wrench  or  lever.  The  screw 
is  used  in  vises ;  in  raising  buildings ; 
in  copying  letters,  and  in  presses  for 
squeezing  the  juice  from  apples,  sugar-cane,  etc. 

LAW  OF  EQUILIBRIUM. — When  the  P  is  applied  at  the 
end  of  a  lever,  it  describes  a  circle  of  which  the  lever  is  the 
radius.  The  distance  through  which  the  P  passes,  is  the 
circumference  of  this  circle ;  and  the  height  to  which  the 
W  is  elevated  at  each  revolution  of  the  screw,  is  the  distance 


*  If  we  roll  into  a  wagon  a  barrel  of  pork,  weighing  200  Ibs.,  up  a  plane  12  feet 
long  and  3  feet  high,  we  have  x  =  50  Ibs.  :  200  Ibs.  : :  3  feet :  12  feet  We  lift  only 
50  Ibs.,  or  I  of  the  barrel,  but  we  raise  it  through  four  times  the  space  that  would 
be  necessary  if  we  could  elevate  it  directly  into  the  wagon.  We  thus  lose  speed 
and  gain  power.  The  longer  the  inclined  plane,  the  heavier  the  load  we  can  lift,  but 
the  more  time  it  will  take  to  do  it. 

t  Near  Lake  Lucerne  is  a  forest  of  firs  on  the  top  of  Mount  Pilatus,  an  almost 
inaccessible  Alpine  summit.  By  means  of  a  wooden  trough,  the  trees  are  conducted 
into  the  water  below,  a  distance  of  eight  miles,  in  as  many  minutes.  One  standing 
near  hears  a  roar  as  of  distant  thunder,  and  the  next  instant  the  descending  tree 
darts  past  and  plunges  downward  out  of  sight.  The  force  with  which  it  falls  IB  so 
prodigious,  that  if  it  jumps  out  of  the  trough  it  is  dashed  to  pieces. 


76 


ELEMENTS    OF    MACHINES. 


FIG.  56. 


between  two  of  the  threads.  Applying  the  law  of  mechanics, 
P  x  circumference  of  circle  =  W  x  interval  between  the 
threads ;  hence, 

P  :  W  : :  interval  :  circumference.  (6.) 

The  efficiency  of  the  screw  may  be  increased  by  length- 
ening the  lever,  or  by  diminishing  the  dis- 
tance between  the  threads. 

5.  The  Wedge  consists  generally  of 
two  inclined  planes  placed  back  to  back. 
It  is  used  for  splitting  wood  and  stone 
and  lifting  vessels  in  the  dock.  Leaning 
chimneys  have  been  righted  by  wedges 
driven  in  on  the  lower  side.  Nails,  needles, 
pins,  knives,  axes,  etc.,  are  made  on  the 
principle  of  the  wedge. 

The  LAW  OF  EQUILIBKIUM  is  the  same  as  that  of  the  in- 
clined plane — viz.,* 

P  :  W   :  :  thickness  of  wedge  :  length  of  wedge.  (7.) 

6.  The  Pulley  consists  of  a  wheel,  within  the  grooved 
edge  of  which  runs  a  cord. 
A  FIXED  PULLEY  (Fig.  57)  merely  changes 
the  direction  of  the  force.  There 
is  no  gain  of  power  or  speed,  as 
the  hand  P  must  move  down  as 
much  as  the  weight  W  rises,  and 
both  with  the  same  velocity.  It 
is  simply  a  lever  of  the  first  class 
with  equal  arms.  By  its  use  a 
man  standing  on  the  ground  will 

hoist  a  flag  to  the  top  of  a  lofty 
pole,  and  thus  avoid  the  trouble 
and  danger  of  climbing  up  with 
it/  Two  fixed  pulleys,  arranged 
as  shown  in  Fig.  58,  enable  a 


FIG.  57. 


FIG.  58. 


*  In  practice,  however,  this  by  no  means  accounts  for  the  prodigious  power  of  the 
wedge.    Friction,  in  the  other  mechanical  powers,  diminishes  their  efficiency ;  in 


THE    PULLEY. 


77 


PIG.  59. 


heavy  load  to  be  elevated  to  the  upper  story  of  a  building  by 
horse-power. 

A  MOVABLE  PULLEY  is  represented  in  Fig.  59.  One- 
half  of  the  barrel  is  sustained  by  the  hook  while 
the  hand  lifts  the  other.  As  the  P  is 
one-half  the  W,  it  must  move  through 
twice  the  space ;  in  other  words,  by 
taking  twice  the  time,  we  can  lift  twice 
as  much.  Thus  power  is  gained  and 
time  lost. 

We  may  also  explain  the  single  mov- 
able pulley  by  Fig.  60.  A  represents 
the  F,  R  the  W  acting  in  the  line  OR, 
and  B  the  P  acting  in  the  line  BP.  This  is  a 
lever  of  the  second  class  ;  and  as  AO  =  J  AB, 
P  =  i  W. 

COMBINATIONS  OF  PULLEYS. — (1.)  In  Fig.  61,  we  have  the 
W  sustained  by  three  cords,  each  of 
which  is  stretched  by  a  tension  equal 
to  the  P;  hence  1  Ib.  of 
power  will  balance  3 
Ibs.  of  weight.  (2.)  In 
Fig.  62,  the  P  will  sus- 
tain a  W  of  4  Ibs.  (3.) 
In  Fig.  63,  the  cord 
marked  1  1  has  a  ten- 
sion equal  to  P  in  each 
part ;  the  one  marked 
2  2  has  a  tension  equal 
to  2P  in  each  part,  and 
so  on  with  the  others. 
The  sum  of  the  ten- 
sions acting  on  W  is  16 ;  hence  "W  =  16  P. 

this  it  is  essential,  else  the  wedge  would  fly  back  and  the  effect  be  lost.  In  the 
others,  the  P  is  applied  as  a  steady  pressure ;  in  this  it  is  a  sadden  blow,  and  depends 
npon  the  striking  force  of  the  hammer. 


FIG.  61. 


FIG.  62. 


FIG.  63. 


78  ELEMENTS    OF    MACHINES. 

In  this  system,  D  rises  twice  as  fast  as  0,  four  times  as  fast 
as  B,  etc.  Work  must  stop  when  D  reaches 
E,  which  gives  little  sweep  to  A  for  lifting 
W.  (4.)  Fig.  64  represents  the  ordinary 
"tackle-block"  used  by  mechanics. 

LAW  OF  EQUILIBRIUM. — When  a  continu- 
ous rope  is  used,  let  n  represent  the  number 
of  separate  parts  of  the  cord  which  sustain 
the  movable  block.  We  then  have 

P=?  (8-) 

When  the  number  of  movable  and  of  fixed 
pulleys  is  equal,  in  general,  W  =  P  x  twice 
the  number  of  movable  pulleys. 

7.  Cumulative  Contrivances. — A  hammer,  club,  pile- 
driver,  sling,  fly-wheel,  etc.,  are  instruments  for  accumu- 
lating energy  to  be  used  at  the  proper  moment.     Thus  we 
may  press  a  hammer  on  the  head  of  a  nail  with  all  our 
strength  to  no  purpose  ;  but  swing  the  hammer  the  length 
of  the  arm,  and  the  blow  will  bury  the  nail  to  the  head. 
The  strength  of  our  muscles  and  the  attraction  of  gravity 
during  the  fall  both  gather  force  to  be  'exerted  at  the  in- 
stant of  contact.     A  fly-wheel  by  its  momentum  equalizes 
an  irregular  force,  or  produces  a  sudden  effect.* 

8.  Perpetual  Motion. — It  is  impracticable  to  make  a 
machine  capable  of  perpetual  motion.     No  combination  can 
produce  energy ;  it  can  only  direct  that  which  is  applied. 
In  all  machinery  there  is  friction ;  this  must  ultimately 
exhaust  the  power  and  bring  the  motion  to  rest.     The  only 
question  is,  how  long  time  will  be  required  for  the  leakage 
to  drain  the  reservoir. 

*  We  see  the  former  illustrated  in  a  sewing-machine,  and  the  latter  in  a  punch 
operated  by  a  treadle.  In  the  one  case,  the  irregular  action  of  the  foot  is  turned  into 
a  uniform  motion,  and  in  the  other  it  is  concentrated  In  a  heavy  blow  that  will  pierce 
a  thick  piece  of  metal 


PRACTICAL    QUESTIONS.  79 

PRACTICAL  QUESTIONS.—!.  Describe  the  rudder  of  a  boat  as  a  lever.  A  door. 
A  door-latch.  A  lemon-squeezer.  A  pitchfork.  A  spade.  A  shovel.  A  sheep- 
shears.  A  poker.  A  pair  of  tongs.  A  balance.  A  pair  of  pincers.  A  wheelbarrow. 
A  man  pushing  open  a  gate  with  his  hand  near  the  hinge.  A  sledge-hammer,  when 
the  arm  is  swung  from  the  shoulder.  A  nut-cracker.  The  arm  (see  Physiology, 
p.  48).  2.  Show  the  change  that  occurs  from  the  second  to  the  third  class  of  lever, 
when  you  take  hold  of  a  ladder  at  one  end  and  raise  it  against  a  building.  3.  Why  is 
a  pinch  from  the  tongs  near  the  hinge  more  severe  than  one  near  the  end  ?  4.  Two 
persons  are  carrying  a  weight  of  250  Ibs.,  hanging  between  them  from  a  pole  10  feet 
in  length.  Where  should  it  be  suspended  so  that  one  will  lift  only  50  Ibs.  ?  5.  In  a 
lever  of  the  first  class,  6  feet  long,  where  should  the  F  be  placed  so  that  a  P  of  1  Ib.  will 
balance  a  W  of  23  Ibs.  ?  6.  What  P  would  be  required  to  lift  a  barrel  of  pork  with  a 
windlass  whose  axle  is  1  foot  in  diameter,  and  handle  3  feet  long  ?  7.  What  sized 
axle,  with  a  wheel  6  feet  in  diameter,  would  be  required  to  balance  a  W  of  one  ton  by 
a  P  of  100  Ibs.  ?  8.  What  number  of  movable  pulleys  would  be  required  to  lift  a  W 
of  200  Ibs.  by  a  P  of  25  Ibs.  ?  9.  How  many  pounds  could  be  lifted  with  a  system  of 
4  movable  pulleys  by  a  P  of  100  Ibs.  ?  10.  What  W  could  be  lifted  with  a  single 
horse-power  *  acting  on  a  system  of  pulleys  shown  in  Fig.  64  ?  11.  What  distance 
should  there  be  between  the  threads  of  a  screw  to  enable  a  P  of  25  Ibs.  acting  on  a 
handle  three  feet  long,  to  lift  a  ton  ?  12.  How  high  would  a  P  of  12  Ibs.,  moving 
16  feet  along  an  inclined  plane,  lift  a  W  of  96  Ibs.  ?  13.  I  wish  to  roll  a  barrel  of  flour 
into  a  wagon,  the  box  of  which  is  four  feet  from  the  ground.  I  can  lift  but  24  Ibs. 
How  long  a  plank  must  I  get  ?  14.  Two  persons,  A  and  B,  at  the  ends  of  a  bar  5  feet 
long  carry  a  load  of  250  pounds.  A  is  stronger  than  B  in  the  ratio  of  3  to  2.  At  what 
distance  from  A  must  the  load  be  suspended?  What  part  of  the  load  does  each 
carry  ?  15.  A  lever  10  feet  long  has  its  fulcrum  at  the  centre.  A  weight  of  20  pounds 
is  applied  at  one  end,  and  30  pounds  at  18  inches  from  the,  same  end.  On  the  other 
side,  30  inches  from  the  end,  is  a  weight  of  50  pounds.  Where  must  another  weight 
of  40  pounds  be  placed  to  produce  equilibrium  ?  16.  What  W  can  be  lifted  with  a  P  of 
100  Ibs.  acting  on  a  screw  having  threads  1  inch  apart,  and  a  handle  4  feet  long? 
17.  What  is  the  object  of  the  balls  often  cast  on  the  ends  of  the  handle  of  the  screw 
used  in  presses  for  copying  letters  ?  18.  In  a  steelyard  2  feet  long,  the  distance  from 
the  weight-hook  to  the  fulcrum-hook  is  2  inches  ;  how  heavy  a  body  can  be  weighed 
with  a  pound  weight  ?  19.  Describe  the  change  from  the  first  to  the  third  class  of 
lever,  in  the  different  ways  of  using  a  pitchfork  or  a  spade.  20.  Why  are  not  black- 
smiths' tongs  and  fire-tongs  constructed  on  the  same  principle  ?  21.  In  a  lever  of  the 
third  class,  what  W  will  a  P  of  50  Ibs.  balance,  if  one  arm  be  12  feet  and  the  other 
8  feet  long  ?  22.  In  a  lever  of  the  second  class,  what  W  will  a  P  of  50  Ibs.  balance, 
with  a  lever  12  feet  long,  and  the  W  3  feet  from  the  F  ?  23.  In  a  lever  of  the  first 
class,  what  W  will  a  P  of  50  Ibs.  balance,  with  a  lever  12  feet  long,  and  the  F  3  feet 
from  the  W  ?  24.  In  a  wheel  and  axle,  the  P  =  40  Ibs.,  ttie  W  =  360  Ibs.,  and  the 
diameter  of  the  axle  =  8  in.  Required  the  circumference  of  the  wheel.  25.  Suppose, 
in  a  wheel  and  axle,  the  P  =  20  Ibs.,  the  W  =  240  Ibs.,  and  the  diameter  of  the  wheel 
=  4  feet.  Required  the  circumference  of  the  axle.  26.  Required,  in  a  wheel  and 
axle,  the  diameter  of  the  wheel,  the  diameter  of  the  axle  being  10  inches,  the  P 
100  Ibs.,  and  the  W  1  ton.  27.  Why  is  the  rim  of  a  fly-wheel  made  so  heavy? 
28.  Describe  the  hammer,  when  used  in  drawing  a  nail,  as  a  dent  lever,  i.  e.,  one  in 
which  the  bar  is  not  straight.  29.  Describe  the  four  levers  shown  in  Fig.  47,  when 
both  the  load  of  hay  and  the  weight  are  considered,  respectively,  as  the  W  and 
the  P. 

*  A  horse-power  is  a  force  which  is  equivalent  to  650  foot-pounds,  i.  e.,  can  raise 
against  gravity  550  Ibs.  one  foot  in  one  second,  or  33,000  Ibs.  one  foot  iu  one  minute. 


80  ELEMENTS    OF    MACHINES. 


SUMMARY. 

All  machines  can  be  resolved  into  one  or  more  elementary  forms 
Of  these  there  are  six,  viz.,  the  lever,  the  wheel  and  axle,  the  inclired 
plane,  the  screw,  the  wedge  and  the  pulley.  Though  called  the 
mechanical  powers,  they  are  only  instruments  by  which  we  can  avail 
ourselves  of  the  forces  of  nature.  Molar  energy  or  the  motion  of 
masses,  as  of  air,  water,  steam,  etc.,  is  thus  utilized,  while  the  strength 
of  a  horse  does  the  work  of  many  men.  A  force  of  small  intensity 
made  to  act  through  a  considerable  distance  becomes  one  of  great  inten- 
sity acting  through  a  small  distance,  and  vice  versa.  No  machine  is  a 
source  of  power,  but  in  all  cases  P  x  Pd  =  W  x  Wd.  The  law  of 
energy  thus  forbids  perpetual  motion.  The  lever  is  a  bar  resting  at 
some  part  on  a  prop  as  a  centre  of  motion.  To  this  simple  machine 
may  be  reduced  also  the  wheel  and  axle,  and  the  pulley.  The  crow- 
bar, claw-hammer  for  drawing  nails,  pincers,  windlass  and  steelyard 
are  examples  of  the  various  classes  of  levers.  To  the  inclined  plane 
may  be  reduced  also  the  wedge  and  the  screw.  The  laws  of  falling 
bodies  obtain  so  that  on  a  plane  sloping  one  foot  in  sixteen  a  body 
(neglecting  friction)  would  descend  only  one  foot  in  the  first  second. 
The  awl,  vise,  carpenter's  plane,  corkscrew,  tackle-block  and  stairs  are 
common  modifications  of  the  inclined  plane. 


HISTORICAL    SKETCH. 

Simple  machines  for  moving  large  bodies  are  as  old  as  history. 
The  Babylonians  knew  the  use  of  the  lever,  the  pulley  and  the  roller. 
The  Romans  were  acquainted  with  the  lever,  the  wheel  and  axle,  and 
the  pulley  (simple  and  compound).  The  Egyptians,  it  is  thought, 
raised  the  immense  stones  used  in  building  the  Pyramids,  by  inclined 
planes  made  of  earth  which  was  afterward  removed.  Archimedes  in 
the  3d  century  B.  C.,  discovered  the  law  of  equilibrium  in  the  lever.  * 
His  investigations,  however,  were  too  far  in  advance  of  his  time  to 
be  fully  understood,  and  the  teachings  of  Aristotle  were  long  after 
accepted  by  scientific  men.  The  law  of  mechanics  or  of  Virtual 
Velocity,  as  it  is  called,  was  discovered  by  Galileo. 

*  It  is  often  said  that  Archimedes,  in  allusion  to  the  tremendous  power  of  the 
lever,  asserted  that,  Give  him  a  fulcrum  and  he  could  move  the  world.  Had  he  been 
allowed  such  a  chance,  "  the  fulcrum  being  nine  thousand  leagues  from  the  centre  of 
the  earth,  with  a  power  of  200  Ibs.  the  geometer  would  have  required  a  lever  12 
quadrillions  of  miles  long  and  the  power  would  have  needed  to  move  at  the  rate  of  a 
cannon-ball  to  lift  the  earth  one  inch  in  27  trillions  of  vcars." 


V. 

OF  LIQUIDS 


"  The  waves  that  moan  along  the  skore9 

The  "winds  that  sigh  in  blowing, 
Are  sent  to  teach  a  mystic  lore 

Which  men  are  vnx  in.  knowing" 


ANALYSIS. 


r  1.  LIQUIDS  INFLU-   f  (i.)  Law  of  Transmission. 

ENCED    BY    Ex-  J   fa)  Water  as  a  Mechanical  Power. 

TERNAL     PRES-   I  (3.)  Hydrostatic  Press. 

SURE  ONLY.               V- 

CO* 

'  (1.)  Four  Laws  of  Equilibrium. 

O 

(2.)  Rules  for  Computing  Pressure. 

£ 

(3.)  Water 

(a.)  Definition  and  illus- 
tration. 

CO 

O   - 

Level. 

(b.)  Buoyant    force     of 

tr 

2.  LIQUIDS  INFLU- 

(c.)  To  find  'specific  grav- 

O 

X 

ENCED  BY  GRAY-  < 

ITY. 

(4.)  Specific  " 
Gravity. 

ity  of  a  solid. 
(d.)  To  find  specific  grav- 
ity of  a  liquid. 
(e.)  To  find    weight  of 
given  bulk. 

(f.)  To  find  bulk  of  given 

weight. 

(g.)  To  find  vol.  of  a  body. 
__  (n.)  Floating  bodies. 

— DEFINITION  AND  GENERAL  PRINCIPLES. 

f  (1.)  The  velocity  that  of  afallingbody. 
RULES  CONCERN-   '  v~ <  —    -    -  -  J 


1. 


\  2. 
1  3. 


VJ.)  To  find  the  velocity. 

(3.)  To  find  the  quantity. 
EFFECT  OF  TUBES. 
FLOW  OF  WATER  IN  RIVERS. 

r  (1.)  Overshot. 

WATER-WHEELS.^  ^  Breast. 
WAVES.  I  (4.)  Turbine. 

—DEFINITION  AND  GENERAL  PRINCIPLES. 


5. 


1.  AIR  PUMP. 

2.  CONDENSER. 

3.  PROPERTIES 
AIR. 


4.  PRESSURE 
THE  AIR. 


5.  PUMPS. 


OF 


OF 


6.  SIPHON. 

7.  PNEUMATIC  INKSTAND. 

8.  HYDRAULIC  RAM. 

9.  ATOMIZER. 

10.  HEIGHT  OF  THE  Anu 


(1.)  Weight. 
(2.)  Elasticity. 
(3.)  Expansibility. 

(1.)  The  Proof. 

(2.)  Upward  Pressure. 

(3.)  Buoyant  Force. 

(4.)  Amount  of  Pressure. 

(5.)  Pressure  Varies. 

(6.)  Mariotte's  (or  Boyle's)  Law. 

(7.)  Barometer. 

(1.)  Lifting. 

(2.)  Forcing. 

(3.)  Fire-engine. 


I.     HYDROSTATICS. 


FIG.  65. 


Hydrostatics  treats  of  liquids  at  rest.  Its  principles 
apply  to  all  liquids  ;  but  water,  on  account  of  its  abundance, 
is  taken  as  the  type. 

1.  Liquids  Influenced  by  External  Pressure  only. 

—(1.)  LIQUIDS  TRANSMIT  PKESSUEE  EQUALLY  IN  ALL  DIREC- 
TION'S. As  the  particles  of  a  liquid  move 
freely  among  themselves,  there  is  no  loss  by 
friction,  and  any  force  will  be  transmitted 
equally  upward,  downward,  and  sidewise. 
Thus  if  a  bottle  be  filled  with  water  and  a 
pressure  of  1  Ib.  be  applied  upon  the  cork, 
it  will  be  communicated  from  particle  to 
particle  throughout  the  water.  If  the  area 
of  the  cork  be  one  square  inch,  the  pressure 
upon  any  square  inch  of  the  glass  at  n,  a,  b, 
or  c,  will  be  1  Ib.  If  the  inside  surface  of  the 
bottle  be  100  square  inches,  a  pressure  of 
1  Ib.  upon  the  cork  will  produce  a  force 
of  100  Ibs.,  tending  to  burst  the  bottle. 
Illustrations. — The  transmission  of  pres- 
sure by  liquids  under 

FIG.  66. 
A  B 

.P 


some  circumstances,  is 
more  perfect  than  by 
solids.  Let  a  straight 
tube,  AB,  be  filled  with 

a  cylinder  of  lead,  and  a  piston  be  fitted  to  the  end  of  the 
tube.  If  a  force  be  ap- 
plied at  P  it  will  be 
transmitted  to  0  with- 
out loss.  If  instead,  we 
use  a  bent  tube,  the 
force  will  be  transmitted 
in  the  lines  of  the  ar- 
rows, and  will  act  on  P  but  slightly. 


Fio.  67. 


If,  however,  we  fill  the 


84 


PBESSUEE    OF   LIQUIDS    AKD    GASES. 


Fxe. 


FIG.  69. 


tube  with  water,  the  force  will  pass  without  diminution.* 

Take  a  glass  bulb  and 
stem  full  of  water,  as  in 
Fig.  68.  f  If  you  are 
careful  to  let  the  stem 
slip  loosely  through  your 
fingers  as  the  bulb 
strikes,  you  may  pound 
it  upon  a  smooth  sur- 
face with  all  your 
strength.  The  force  of 
the  blow  being  instantly 
transmitted  from  the 
thin  glass  to  the  water, 
which  is  almost  incompressible,  makes  the  bulb  nearly 
as  solid  as  a  ball  of  iron. 
If  a  Rupert's  drop  be  held 
in  a  vial  of  water  so  as  not 
to  touch  the  glass  (Fig.  69), 
and  the  tapering  end  be 
broken,  the  water  will  trans- 
mit the  concussion  and  shat- 
ter the  vial. 

(2.)  WATER  ASA  MECHAN- 
ICAL POWER. — Take  two  cyl- 
inders, P  and  p,  (Fig.  70) 

fitted  with  pistons  and  filled  with  water.     Let  the  area  of  p 

be  2  inches  and  that  of  P  be 
100  inches.  Then,  accord- 
ing to  the  principle  of  the 
equal  pressure  of  liquids,  a 
downward  pressure  of  1  Ib. 
on  each  square  inch  of  the 
small  piston  will  produce  an 
upward  pressure  of  1  Ib. 

*  With  cords,  pulleys,  levers,  etc.,  we  always  lose  about  one-half  of  the  force  by 
friction ;  but  this  "  liquid  rope  "  transmits  it  with  no  appreciable  loss, 
t  The  process  oi  filling  such  bulbs  is  shown  on  p.  186.    They  are  cheaply  pur- 


FIQ.  70. 


HYDROSTATICS. 


85 


on  each  square  inch  of  the  large  piston.      Hence  a  P  of 
2  Ibs.  will  lift  a  W  of  100  Ibs.* 

(3.)   THE  HYDROSTATIC,  OR  HYDRAULIC  PRESS  (Fig.  71) 
utilizes  the  principle  just  explained.     As  the  workman  de- 


FIG.  71. 


presses  the  lever  0,  he  forces  down  the  piston  a  upon  the 
water  in  the  cylinder  A.      The  pressure    is   transmitted 


chased  of  apparatus  dealers  and  explain  not  only  this  point  but  also  the  method  of 
filling  thermometers. 

*  Pascal  announced  the  discovery  of  this  principle  in  the  following  words,  "  If  a 
vessel  full  of  water  closed  on  all  sides  has  two  openings,  the  one  a  hundred  times  as 
large  as  the  other,  and  if  each  be  supplied  with  a  piston  which  fits  exactly,  a  man 
pushing  the  small  piston  will  exert  a  force  which  will  balance  that  of  a  hundred  men 
pushing  the  large  one." 


86  PRESSURE    OF    LIQUIDS    AKD    GASES. 

through  the  bent  tube  of  water  d  under  the  piston  0,  which 
lifts  up  the  platform  K,  and  compresses  the  bales.  If  the 
area  of  a  be  1  inch  and  that  of  0  100  inches,  a  force  of 
100  Ibs.  will  lift  10,000  Ibs.  Still  farther  to  increase  the 
efficiency  of  this  press,  the  handle  is  a  lever  of  the  second 
class.  If  the  distance  of  the  hand  from  the  pivot  be  ten 
times  that  of  the  piston,  a  P  of  100  Ibs.  will  produce  a  force 
of  1,000  Ibs.  at  a.  This  will  become  100,000  Ibs.  at  0.* 
According  to  the  principle  of  mechanics,  PxPd  =  W  x  Wd, 
the  platform  will  ascend  y^  of  the  distance  the  hand  de- 
scends. This  machine  is  used  for  baling  hay  and  cotton, 
for  launching  vessels,  and  for  testing  the  strength  of  ropes, 
chains,  etc. 

2.  Liquids  Influenced  by  Gravity. — The  lower  part 
of  a  vessel  of  water  must  bear  the  weight  of  the  upper  part. 
Thus  each  particle  of  water  at  rest  is  pressed  downward  by 
the  weight  of  the  minute  column  it  sustains.  It  must,  in 
turn,  press  in  every  direction  with  the  same  force,  else  it 
would  be  driven  out  of  its  place  and  the  liquid  would  no 
longer  be  at  rest.  Indeed,  when  a  liquid  is  disturbed  it  comes 
to  rest — i.  e.,  there  is  an  equilibrium  established — only  when 
this  equality  of  pressure  is  produced.  In  consequence  of 
this  constant  pressure  the  following  laws  obtain  : 

(1.)  FOUR  LAWS  OF  EQUILIBRIUM. — I.  Liquids  at  rest 
press  downward,  upward  and  sidewise  with  the  same  force. 
If  the  series  of  glass  tubes  shewn  in  Fig.  72  be  placed  in 
a  pail  of  water,  the  liquid  will  be  forced  up  1  by  the  upward 
pressure  of  the  water,  2  by  the  downward  pressure,  3  by  the 
lateral  pressure,  and  4  by  the  three  combined  in  different 
portions  of  the  tube.  The  water  will  rise  in  them  all  to  the 
same  height — i.  e.,  to  the  level  of  the  water  in  the  pail. 


*  The  presses  employed  for  raising  the  immense  tubes  of  the  Britannia  Bridge 
across  the  Menai  Strait,  were  each  capable  of  lifting  2,672  tons,  and  of  "  throwing 
water  in  a  vacuum  to  a  height  of  nearly  six  miles,  or  over  the  top  of  the  highest 
mountain."  The  difference  between  a  and  C  may  be  increased  until  the  weight  of  a 
girl's  hand  would  lift  a  man-of-war. 


HYDROSTATICS. 


87 


II.  The  pressure  increases  with  the  depth.     The  pressure 
per  square  foot  at  the  depth  of  1  foot  is  the  weight  of  a 
cubic  foot  of  water — viz.,  about  62£  Ibs.  (1,000  ozs.)  ;  at 
2  feet,  twice  that  amount, ;  and  so  on.     In  sea-water  it  is 
greater,  as  that  weighs  64.37  Ibs.  per  cubic  foot.     At  great 
depths  this  pressure  becomes  enormous.    If  a  square  glass 
bottle,  empty  and  firmly  corked,  be  sunk  into  the  water,  it 
will  generally  be  crushed  before  sinking  ten  fathoms.    When 
a  ship  founders  at  sea, 

the    water    is    forced  FlG- 73- 

into  the  pores  of  the 

wood,  so  that  no  part 

can  ever  rise  again  to 

the  surface  to   reveal 

the  fate    of   the    lost 

vessel. 

III.  The    pressure 
does  not  depend  on  the 

shape  or  size  of  the  vessel.  In 
the  apparatus  shown  in  Fig.  73 
the  water  rises  to  the  same  height 
in  the  various  tubes,  which  com- 
municate with  one  another. 

The  Hydrostatic  Bellows  con- 
sists of  two  boards,  each  hinged 

on  one  side  and  resting  on  a  rubber  bag,  to  which  is  at- 
tached an  upright  tube,  A.     Water  is  poured  in  at  A  until 


88 


PRESSURE    OF    LIQUIDS    AND    GASES. 


Fio.  74. 

A               B 

\ 

^ 

.  , 

IT. 

]      f\ 

the  bag  and  the  tube  are  filled.     The  pressure  of  the  column 

of  water  in  the  tube  lifts 
the  weights  hung  by 
crossbars  beneath.  It 
will  make  no  difference 
in  the  weight  supported, 
whether  we  use  A  or  B, 
although  the  former 
holds  ten  times  as  much 
water  as  the  latter.  0, 
however,  being  longer, 
the  water  will  exert  a 
greater  pressure.  An- 
other form  of  the  appa- 
ratus (Fig.  75)  consists 
of  two  boards  connected 
by  a  band  of  leather,  in 
which  a  tall  tube  A  is  inserted.  If  this  be  filled  with  water, 
the  pressure  will  lift  a  weight  as 

much  greater  than 

the  weight  of  the 

water  in  the  tube 

as  the  area  of  the 

bellows  -  board    is 

greater  than  that 

of  the  tube.     Ap- 
plying the  law  of 

mechanics,  if  1  oz. 

of   water    raise    a 

weight  of  50  ozs. 

1  inch,  the  water 
must  fall  50  inches. 

A  strong  cask  fitted  with  a  small 
pipe  30  or  40  feet  long,  if  filled  with 
water  will  be  burst  asunder.  *  The 

*  Suppose  the  pipe  to  have  an  area  of  ,'»  square  inch,  and  to  hold  |  Ib.  of  water. 
The  pressure  on  each  ^  of  an  inch  of  the  interior  of  the  cask  would  be  £  Ib.,  or 
Sgjjp  Ibs.  on  each  square  foot — a  pressure  no  common  barrel  could  sustain. 


FIG.  76. 


PIG.  75. 


HYDROSTATICS.  89 

pressure  is  as  great  as  if  the  tube  were  of  the  same  diameter 
as  the  cask.  In  a  coffee-pot  the  small  quantity  of  liquid  in 
the  spout  balances  the  large  quantity  in  the  vessel.  If  it 
were  not  so,  it  would  rise  in  the  spout  and  run  out.* 

IV.    Water  seeks  its  level.    In  Tig.  77  a  tank  is  situated  on 
a  hill,  whence  the  water  is  conducted  underground  through 

FIG.  77. 


a  pipe  to  the  fountain.  In  theory  the  jet  will  rise  to  the 
level  of  the  reservoir,  but  in  practice  it  falls  short,  owing  to 
the  friction  at  the  nozzle  of  the  pipe  and  in  passing  through 
the  air,  and  the  weight  of  the  falling  drops. 

Artesian  Wells. \ — Let  A  B  and  C  D  represent  curved 
strata  of  clay  impervious  to  water,  and  K  K  a  layer  of 
gravel.  The  rain  falling  on  the  hills  filters  down  to  C  D, 

*  The  principle  that  a  small  quantity  of  water  will  thus  balance  another  quantity, 
however  large,  or  will  lift  any  weight,  however  great,  is  frequently  termed  the 
"  Hydrostatic  Paradox."  It  is  only  an  instance  of  a  general  law,  and  is  no  more 
paradoxical  than  the  action  of  the  lever. 

t  They  are  so  named  because  they  have  long  been  used  in  the  province  of  Artois 
(Latin,  Artesium),  France.  They  were,  however,  early  employed  by  the  Chinese  for 
the  purpose  of  procuring  gas  and  salt  water. 


PRESSURE    OF    LIQUIDS    AND    GASES. 
Fra  78. 


and  collects  in  this  basin.  If  a  well  be  bored  at  H,  as  soon 
as  it  readies  the  gravel  the  water  will  rush  upward,  under 
the  tremendous  lateral  pressure,  to  the  height  of  the  source, 
often  spouting  high  in  air.  * 

Wells. — Of  the  rain  which  falls  on  the  land,  a  part  runs 
directly  to  the  streams  and  part  soaks  into  the  soil.  The 
latter  portion  may  filter  down  to  an  impermeable  layer  of 
rock  or  clay,  and  then  run  along  till  it  oozes  out  at  some 
lower  point  as  a  spring  ;  or,  if  it  cannot  escape,  it  will  col- 
lect in  the  ground.  If  a  well  be  sunk  into  this  subterra- 
nean reservoir,  the  water  will  rise  in  it  to  the  level  around,  f 

(2.)  EULES  FOR  COMPUTING  PRESSURE. — I.  To  find  the 
pressure  on  the  bottom  of  a  vessel.  Multiply  the  area  of  the 

*  The  famous  well  at  Grenelle,  Paris,  is  at  the  bottom  of  a  basin  which  extends 
miles  from  the  city.  It  is  about  1,800  feet  deep,  and  furnishes  656  gallons  of  water 
per  minute.  The  two  wells  of  Chicago  are  about  700  feet  deep,  and  discharge  daily 
about  432,000  gallons.  Being  situated  on  the  level  prairie,  the  force  with  which  the 
water  comes  to  the  surface  indicates  that  it  is  supplied  perhaps  from  Rock  River,  100 
miles  distant.  There  are  also  valuable  artesian  weils  at  Louisville,  Kentucky,  and 
at  Charleston,  South  Carolina.  When  the  water  comes  from  a  great  depth  it  is 
generally  warm.  (See  Geolopy,  p.  21.) 

t  "  From  a  forgetfulness  of  this  principle  fhe  company  which  dug  the  Thames 
and  Medway  Canal,  England,  incurred  heavy  damages.  Having  planned  the  canal  to 
be  filled  at  high  tide,  the  salt  water  spread  immediately  into  all  the  wells  of  the  sur- 
rounding region.  Had  the  canal  been  dug  a  few  feet  lower,  the  evil  would  have  been 
avoided."— Arnott. 


HYDROSTATICS.  91 

base  by  the  perpendicular  height,  and  that  product  by  the 
weight  of  a  cubic  foot  of  the  liquid. 

II.  To  find  the  pressure  on  the  side  of  a  vessel.  Multiply 
the  area  of  the  side  by  half  of  the  perpendicular  height,* 
and  that  product  by  the  weight  of  a  cubic  foot  of  the  liquid. 
The  pressure  on  the  bottom  of  a  cubical  vessel  of  water  is 


FIG.  79. 


the  weight  of  the  water  ;  on  each  side,  one-half ;  and  on  the 
four  sides,  twice  the  weight ;  therefore,  on  the  five  sides  the 
pressure  is  three  times  the  weight  of  the  water. 

(3.)  WATEE-LEYEL. — The  surface  of  standing  water  is 
said  to  be  level — i.  e.9  horizontal  to  a  plumb-line.  This  is 
true  for  small  sheets  of  water,  but  for  larger  bodies  an  allow- 
ance must  be  made  for  the  circular  form  of  the  earth  (Fig. 
79).  The  curvature  is  8  inches  per  mile,  and  increases  as 
the  square  of  the  distance,  f 

*  This  clause  of  the  rule  holds  only  when  the  centre  of  gravity  of  the  side  is  at 
half  the  perpendicular  height.  In  general,  the  depth  of  its  centre  of  gravity  below 
the  surface  should  be  used  as  the  multiplier. 

t  For  two  miles  it  is  8  inches  x  22  =  3-2  inches.  If  one's  eye  were  at  the  level  of 
the  water,  he  could  not  see  an  object  60  feet  high  at  a  distance  of  10  miles. 


92 


PRESSURE    OF    LIQUIDS    AND    GASES. 


FIG.  80. 


FIG.  81. 


The  spirit-level  is  an  instrument  used  by  builders  for  lev- 
elling. It  con- 
sists of  a  slightly- 
curyed  glass  tube 
so  nearly  full  of 
alcohol  that  it 
holds  only  a  bub- 
ble of  air.  When 

the  level  is  horizontal,  the  bubble  remains  at  the  centre  of 

the  tube. 

(4.)  SPECIFIC  GRAVITY,  or  relative  weight,  is  the  weight 
of  a  substance  compared  with  that  of  the  same  bulk 
of  another  substance.  It  shows  the  relative  mass  or  the 
density  of  a  body.  Water  is  taken  as  the  standard*  for 
solids  and  liquids,  and  air  for  gases. 
A  cubic  inch  of  sulphur  weighs 
twice  as  much  as  a  cubic  inch  of 
water ;  hence  its  specific  gravity 
=  2.  A  cubic  inch  of  carbonic- 
acid  gas  weighs  1.52  times  as  much 
as  the  same  volume  of  air ;  hence 
its  specific  gravity  =  1.52. 

Buoyant  Force  of  Liquids. — The 
cube  abed  is  immersed  in  water. 
The  lateral  pressure  at  a  is  equal 
to  that  at  5,  because  both  sides  are 
at  the  same  depth  ;  hence  the  body 
has  no  tendency  toward  either  side  of  the  jar.  The  upward 

*  The  water  must  be  at  39.2°  F.,  its  greatest  density.  In  all  exact  measurements, 
especially  of  standards,  it  is  necessary  to  know  the  temperature.  For  the  scale  that 
is  a  foot  long  to-day  may  he  more  or  less  than  a  foot  long  to  morrow ;  the  measure 
that  holds  a  pint  to-day  may  hold  more  or  less  than  a  pint  to-morrow.  Nay,  more, 
these  measures  may  not  be  the  same  in  two  consecutive  moments.  When  a  car- 
penter takes  up  his  rule  and  applies  it  to  some  object,  the  size  of  which  he  wishes  to 
determine,  it  becomes  in  that  instant  longer  than  it  was  before ;  when  a  druggist 
grasps  his  measuring  glass  in  his  hand  to  dispense  some  of  his  preparations,  the 
glass  increases  in  size.  A  person  enters  a  cool  room,  and  at  once  it  becomes  more 
capacious,  for  its  walls,  ceiling  and  floor,  because  of  the  heat  he  imparts,  immediately 
expand.— Draper. 


HYDROSTATICS.  93 

pressure  at  c  is  greater  than  the  downward  pressure  at  d, 
because  its  depth  is  greater  ;  hence  the  cube  has  a  tendency 
to  rise.  This  upward  pressure  is  called  the  buoyant  force 
of  the  water.  It  is  equal  to  the  weight  of  the  liquid  dis- 
placed. For  the  downward  pressure  at  d  is  the  weight  of 
a  column  of  water  whose  area  is  that  of  the  top  of  the  cube, 
and  whose  perpendicular  height  is  n  d,  and  the  upward 
pressure  at  c  is  equal  to  the  weight  of  a  column  of  the  same 
size  whose  perpendicular  height  is  c  n.  The  difference 
between  the  tivo,  or  the 
buoyant  force,  is  the 
weight  of  a  bulk  of 
water  equal  to  the  size 
of  the  cube. 

The   same  principle 
is  shown  in  the  "cyl- 
inder-and-bucket      ex- 
periment."     The   cyl- 
inder a  exactly  fits  in 
the  bucket  I.     When 
the  glass  vessel  in  which 
the   cylinder  hangs   is 
empty,  the  apparatus  is 
balanced  by 
weights  placed 
in    the    scale- 
pan.      Next, 
water  is  poured 

into  the  glass  vessel.  Its  buoyant  force  raises  the  cylinder 
and  depresses  the  opposite  scale-pan.  Then  water  is  dropped 
into  the  bucket ;  when  it  is  exactly  full,  the  scales  will 
balance  again.  This  proves  that  "a  body  in  water  is 
buoyed  up  by  a  force  equal  to  the  weight  of  the  water  it 
displaces." — Archimedes9  law,  p.  119. 

To  find  the  specific  gravity  of  a  solid.  "Weigh  the  body  in 
air,  and  in  water  ;  the  difference  is  the  weight  of  its  volume 
of  water;  divide  its  weight  in  air  by  its  loss  of  Wight  in 


94 


PRESSURE    OF    LIQUIDS    AND    GASES. 


FIG.  83. 


water  ;  the  quotient  is  the  specific  gravity.  Thus,  sulphur 
loses  one-half  its  weight  when  immersed  in  water  ;  hence  it 
is  twice  as  heavy  as  water,  and  its  specific  gravity  =  2.* 

To  find  the  specific  gravity  of  a  liquid  ly  the  specific-gravity 
flask.  This  is  a  bottle  which  holds  exactly  1,000  grains  of 
water.  If  it  will  hold  1,840  grains  of  sulphuric  acid,  the 
specific  gravity  of  the  acid  is  1.84. 

To  find  the  specific  gravity  of  a  liquid  ly  a  hydrometer. 
This  instrument  consists  of  a  glass  tube,  closed  at  one  end 
and  having  at  the  other  a  bulb  containing  mercury.  A 
graduated  scale  is  marked  upon  the  tube. 
The  alcoholmeter ',  used  in  testing  alcohol,  is 
so  balanced  as  to  sink  in  pure  water  to  the 
zero  point.  As  alcohol  is  lighter  than  water, 
the  instrument  will  descend  for  every  ad- 
dition of  spirits.  The  degrees  of  the  scale 
indicate  the  percentage  of  alcohol.  Similar 
instruments  are  used  for  determining  the 
strength  of  milk,  acids,  etc. 

To  find  the  weight  of  a  given  volume  of  any 
substance.  Multiply  the  weight  of  one  cubic 
foot  of  water  by  the  specific  gravity  of  the 
substance,  and  that  product  by  the  number 
of  cubic  feet.  Ex. :  What  is  the  weight 
of  three  cubic  feet  of  cork  ?  Solution  : 
1,000  ozs.  x  .240  f  =  240  ozs. ;  240  ozs.  x  3  =  720  ozs. 

*  If  the  body  will  not  sink  in  water  attach  it  to  a  heavy  body.  1.  Weigh  the 
lighter  body  in  air  (A).  2.  Weigh  the  heavy  body  in  water  (B).  3.  Weigh  both 
together  in  water  (O.  Now  C  ie  less  than  B  because  the  light  body  buoys  up  the 
heavy  one ;  i.  e.,  all  its  downward  weight  A  is  lost,  and  is  actually  converted  into  an 
upward  or  lifting  force  =  B—  C.  Therefore  the  loss  of  the  light  body  in  water  = 

A+B—C.'.  spec.grav.  =  - — - — -. 

A  +  Jtf  —  (/ 

t  TABLE  OP  SPECIFIC  GBAVITT.    (See  Chem.,  p.  288.) 


Iridium ...  31.80|Zinc 7.15 

Platinum 21.53  |  Diamond about  3.50 

Gold...  ..  19.34 


Mercury 13.59 

..  11.36 


Flint  Glass 2.76 

Chalk 2.65 

Sulphur 2.00 


Lead 

Silver 10.50    Ice 93 

Copper 8.90  I  Potassium 86 

Cast-iron 7.21    Quicklime .80 


PineWood 66 

Cork 24 

Sulphuric  Acid 1.84 

Water  from  Dead  Sea.  1.24 

Milk 1.03 

Sea-water 1.03 

Absolute  Alcohol...        .79 


HYDROSTATICS. 


95 


To  find  the  volume  of  a  given  weight  of  any  substance. 
Multiply  the  weight  of  a  cubic  foot  of  f  water  by  the  specific 
gravity  of  the  substance,  and  divide  the  given  weight  by 
that  product.  The  quotient  is  the  required  volume  in  cubic 
feet.  Ex.:  What  is  the  volume  of  20,000  ozs.  of  lead? 
Solution:  1,000  ozs. x  11.36  =  11,360 ;  20,000-^11,360  = 
1.76+  cu.  ft. 

To  find  the  volume  of  a  body.  Weigh  it  in  water.  The 
loss  of  weight  is  the  weight  of  the  displaced  water.  Then, 
as  a  cubic  foot  of  water  weighs  1,000  ozs.,  we  can  easily  find 
the  volume  of  water  displaced.  Ex. :  A  body  loses  10  oz.  on 
being  weighed  in  water.  The  displaced  water  weighs  10  oz. 
and  is  -^  of  a  cubic  foot ;  this  is  the  exact  volume  of  the 
body. 

Floating  Bodies. — A  body  will  float  in  water  when  its 
weight  is  not  greater  than  that  of  an  equal  volume  of  the 
liquid,  and  its  weight  always  equals 
that  of  the  fluid  displaced.  An  egg 
dropped  into  a  glass  jar  half-full  of 
water  (Eig.  84)  sinks  directly  to  the 
bottom.  If,  by  means  of  a  funnel 
with  a  long  tube,  we  pour  brine 
beneath  the  water,  the  egg  will  rise. 
We  may  vary  the  experiment  by  not 
dropping  in  the  egg  until  we  have 
half  filled  the  jar  with  the  brine. 
The  egg  will  then  fall  to  the  centre,  and  there 
float.  Almost  any  solid  if  dissolved  in  water 
fills  the  pores  of  the  water  without  adding  to  its 
volume.  This  increases  its  density  and  buoy- 
ant power.  A  person  can  therefore  swim  more 
easily  in  salt  than  in  fresh  water.* — An  iron 
ship  will  not  only  float  itself,  but  also  carry  a 


FIG.  84 


*  Bayard  Taylor  says  that  he  could  float  on  the  surface  of  the  Dead  Sea,  with  a 
log  of  wood  for  a  pillow,  as  comfortably  as  if  lying  on  a  spring  mattress.  Another 
traveller  remarks,  that  on  plunging  in  he  was  thrown  out  again  like  a  cork ;  and 
that  on  emerging  and  drying  himself,  the  crystals  of  salt  which  covered  his  body 
made  him  resemble  an  **  animated  stick  of  rock-candy." 


96  PRESSURE   OF   LIQUIDS   AND   GASES. 

heavy  cargo,  because  it  displaces  a  great  volume  of  water.— 
A  body  floating  in  water  has  its  centre  of  gravity  at  the 
lowest  point,  when  it  is  in  stable  equilibrium.* — Fishes  have 
air-bladders,  by  which  they  can  rise  or  sink  at  pleasure,  f 

PRACTICAL  QUESTIONS.— 1.  Why  can  housekeepers  test  the  strength  of  lye, 
by  trying  whether  or  not  an  egg  will  float  on  it  ?  2,  How  much  water  will  it  tajie  to 
make  a  gallon  of  strong  brine  ?  3.  Why  can  a  fat  man  swim  easier  than  a  lean  one  ? 
4.  Why  does  the  firing  of  a  cannon  sometimes  bring  to  the  surface  the  body  of  a 
drowned  person  ?  Am.  Because  by  the  concussion  it  shakes  the  body  loose  from 
the  mud  or  any  object  with  which  it  is  entangled.  5.  Why  does  the  body  of  a  drowned 
person  generally  come  to  the  surface  of  the  water,  after  a  time  ?  Ans.  Because  the 
gases  which  are  generated  by  decomposition  in  the  body  render  it  lighter.  6.  If  we 
let  bubbles  of  air  pass  up  through  a  glass  of  water,  why  will  they  become  larger  as 
they  ascend  ?  7.  What  is  the  pressure  on  a  lock-gate  14  feet  high  and  10  feet  wide, 
when  the  lock  is  full  of  water  ?  8.  Will  a  pail  of  water  weigh  any  more  with  a  live 
fish  in  it  than  without  ?  9.  If  the  water  filtering  down  through  a  rock  should  collect 
in  a  crevice  an  inch  square  and  250  feet  high,  opening  at  the  bottom  into  a  closed 
fissure  having  20  square  feet  of  surface,  what  would  be  the  total  pressure  tending  to 
burst  the  rock  ?  10.  Why  can  stones  in  water  be  moved  so  much  more  easily  than 
on  land  ?  11.  Why  is  it  so  difficult  to  wade  in  water  when  there  is  any  current  ?  12. 
Why  is  a  mill-dam  or  canal  embankment  small  at  the  top  and  large  at  the  bottom  ? 
13.  In  digging  canals  and  building  railroads,  ought  not  the  engineer  to  take  into  con- 
sideration the  curvature  of  the  earth?  14.  Is  the  water  at  the  bottom  of  the  ocean 
denser  than  at  the  surface  ?  15.  Why  does  the  bubble  of  air  in  a  spirit-level  move  as 
the  instrument  is  turned  ?  16.  Can  a  swimmer  tread  on  pieces  of  glass  at  the  bottom 
of  the  water  more  safely  than  on  land  ?  17.  Will  a  vessel  draw  more  water  in  a 
river  than  in  the  ocean?  18.  Will  iron  sink  in  mercury  ?  19.  The  water  in  the 
reservoir  in  New  York  is  about  80  feet  above  the  fountain  in  the  City  Hall  Park. 
What  is  the  pressure  upon  a  single  inch  of  the  pipe  at  the  latter  point?  20.  Why 
does  cream  rise  on  milk  ?  21.  If  a  ship  founder  at  sea,  to  what  depth  will  it  descend?  :£ 

22.  There  is  a  story  told  of  a  Chinese  boy  who  accidentally  dropped  his  ball  into  a  deep 
hole  where  he  could  not  reach  it.    He  filled  the  hole  with  water,  but  the  ball  would 
not  quite  float.    He  finally  thought  of  a  successful  expedient.    Can  you  guess  it? 

23.  Which  has  the  greater  buoyant  force,  water  or  oil  ?    24.  What  is  the  weight  of 
four  cubic  feet  of  cork  ?     25.  How  many  ounces  of  iron  will  a  cubic  foot  of  cork  float 
in  water  ?    26.  What  is  the  specific  gravity  of  a  body  whose  weight  in  air  is  30  grs. 
and  in  water  20  grs.  ?    How  much  is  it  heavier  than  water  ?    27.  Which  is  heavier, 


*  Herschel  tells  an  amusing  story  of  a  man  who  attempted  to  walk  on  water  by 
means  of  bulky  cork  boots.  Scarcely,  however,  had  he  ventured  out  ere  the  law  of 
gravitation  seized  him,  and  ah1  that  could  be  seen  was  a  pair  of  heels,  whose  move- 
ments manifested  a  great  state  of  uneasiness  in  the  human  appendage  below. 

t  It  was  formerly  thought  that  a  fish  in  water  has  no  weight.  It  is  said  that 
Charles  II.  of  England  once  asked  the  philosophers  of  his  time  to  explain  this  phe- 
nomenon. They  offered  many  wise  conjectures,  but  no  one  thought  of  trying  the 
experiment.  At  last  a  simple-minded  man  balanced  a  vessel  of  water,  and  on  adding 
a  fish,  found  it  weighed  just  as  much  as  if  placed  on  a  dry  scale-pan. 

$  It  is  a  poetical  thought  that  ships  may  thus  sink  into  submarine  currents  and 
be  carried  hither  and  thither  with  their  precious  cargoes  of  freight  and  passengers, 
on  voyages  that  know  no  end  and  toward  harbors  that  they  never  reach. 


HYDROSTATICS.  9? 

a  pail  of  fresh  or  one  of  salt  water?  28.  The  weights  of  a  piece  of  syenite-rock  in 
air  and  water  were  3941.8  grs.  and  2607.5  grs.  Find  its  specific  gravity.  29.  A 
specimen  of  green  sapphire  from  Siam  weighed  in  air  21.45  grs.  and  in  water  16.33 
grs. ;  required  its  specific  gravity.  30.  A  specimen  of  granite  weighs  in  air  534,8 
grs  and  in  water  334.6  grs. ;  what  is  its  specific  gravity?  31.  What  is  the  bulk 
of  a  ton  of  iron  ?  A  ton  of  gold  ?  A  ton  of  copper  ?  32.  What  is  the  weight  of  a 
cube  of  gold  4  feet  on  each  side  ?  33  A  cistern  is  12  feet  long,  6  feet  wide,  and 
10  feet  deep ;  when  full  of  water,  what  is  the  pressure  on  each  side  ?  34.  Why  does 
a  dead  fish  always  float  on  its  back  ?  35.  A  given  bulk  of  water  weighs  62.5  grs. 
and  the  same  bulk  of  muriatic  acid  75  grs.  What  is  the  specific  gravity  of  the 
acid  ?  36.  A  vessel  holds  10  Ibs.  of  water ;  how  much  mercury  would  it  contain  ? 
37.  A  stone  weighs  70  Ibs.  in  air  and  50  in  water;  what  is  its  bulk  ?  38.  A  hollow 
ball  of  iron  weighs  10  Ibs. ;  what  must  be  its  bulk  to  float  in  water?  |^§9.  Suppose 
that  Hiero's  crown  was  an  alloy  of  silver  and  gold,  and  weighed  22  ozs.  in  air  and 
20J  ozs.  in  water.  What  was  the  proportion  of  each  metal?  40.  WThy  will  oil, 
which  floats  on  water,  sink  in  alcohol?  41.  A  specific  gravity  bottle  holds  100  gms. 
of  water  and  180  gms.  of  sulphuric  acid.  Required  the  density  of  the  acid.  42.  What 
is  the  density  of  a  body  which  weighs  58  gms.  in  air  and  46  gms.  in  water  ?/  43.  What 
is  the  density  of  a  body  which  weighs  63  gms.  in  air  and  35  gms.  in  a  liquid  of  a 
density  of  .85? 


II.     HYDRAULICS. 

Hydraulics  treats  of  liquids  in  motion.  In  this,  as  in 
Hydrostatics,  water  is  taken  as  the  type.  In.  theory,  its 
principles  are  those  of  falling  bodies,  but  in  practice  they 
cannot  be  relied  upon  except  when  verified  by  experiment. 
The  discrepancy  arises  from  changes  of  temperature  which 
yary  the  fluidity  of  the  liquid,  from  friction,  the  shape  of 
the  orifice,  etc. 

1.  Rules  Concerning  a  Jet. — (1.)  THE  VELOCITY  OF 

A  JET    IS    THE    SAME   AS    THAT    OF  A    BODY  FALLING    FROM 

THE  SURFACE  OF  THE  WATER.  We  can  see  that  this  must 
be  so,  if  we  recall  two  principles  :  First,  "  a  jet  will  rise  to 
the  level  of  its  source  ;  "  and  second,  "  to  elevate  a  body  to 
any  height,  it  must  have  the  same  velocity  that  it  would 
acquire  in  falling  that  distance."  It  follows  that  the 
velocity  of  a  jet  depends  on  the  height  of  the  liquid  above 
the  orifice. 

(2. )    To  Fli^D   THE  VELOCITY  OF  A  JET  OF  WATER,  US6  the 

8th.  equation  of  falling  bodies,  V—  V%  ffd,  in  which  d  is 


98  PRESSURE    OF    LIQUIDS    AND    GASES. 

the  distance  of  the  orifice  below  the  surface  of  the  water. 
Ex.  :  The  depth  of  water  above  the  orifice  is  64  feet ;  re- 
quired the  velocity.  Substituting,  V  =  A/2  x  32  x  64  =  64 
feet. 

(3.)    TO   FIND   THE    QUANTITY   OF  WATER    DISCHARGED   IN 

A  GIVEN  TIME,  multiply  the  area  of  the  orifice  by  the  veloc- 
ity of  the  water,  and  that  product  by  the  number  of  seconds. 
Ex.  :  What  quantity  of  water  will  be  discharged  in  5  seconds 
from  an  orifice  having  an  area  of  -J-  sq.  foot,  at  a  depth  of 
16  feet  ?  At  that  depth,  V  =  VsTxllTxTe  =  32  feet 
per  second ;  multiplying  by  -J-,  we  have  16  cubic  feet  dis- 
charged in  one  second  and  80  cubic  feet  in  five  seconds.* 
In  practice  about  f  of  this  can  be  realized. 

2.  Effect  of  Tubes. — If  we  examine  a  jet  of  water,  we 
see  its  size  is  decreased  just  outside  the  orifice  to  about  two- 
thirds  that  at  the  opening.     This  neck  is  called  the  vena 
contracta,  and  is  caused  by  the  water  producing  cross  cur- 
rents as  it  flows  from  different  directions  toward  the  orifice. 
If  a  tube  of  a  length  twice  or  thrice  the  diameter  of  the 
opening  be  inserted,  the  water  will  adhere  to  the  sides  so 
that  there  will  be  no  contraction,  and  the  flow  be  increased 
to  about  80  per  cent,  of  the  theoretical  amount.     If  the  tube 
be  conical,  and  inserted  with  the  large  end  inward,  the  dis- 
charge may  be  augmented  to  95  per  cent. ;  and  if  the  outer 
end  be  flaring,  it  will  reach  98  per  cent.    Long  tubes  or 
short  angles,  by  friction,  diminish  the  flow  of  water.     It 
is  said  that  an  inch  pipe  200  feet  long  will  discharge  only 
J  as  much  as  a  very  short  pipe  of  the  same  size. 

3.  Flow  of  Water  in  Rivers. — A  fall  of  three  inches 
per  mile  is  sufficient  to  give  motion  to  water,  and  produce  a 
velocity  of  as  many  miles  per  hour.     The  Ganges  descends 
but  800  feet  in  1,800  miles.     Its  waters  require  a  month  to 

*  If,  at  a  foot  below  the  surface,  an  opening  will  furnish  1  gallon  per  minute,  to 
double  that  quantity  the  opening  must  be  4  feet  below  the  top.  Again,  if  a  certain 
power  will  force  through  a  nozzle  of  a  fire-engine  a  given  quantity  of  water  in  a 
minute,  to  double  the  quantity  the  power  must  be  quadrupled  (p.  36). 


HYDRAULICS. 


99 


FIG.  85. 


move  down  this  long  inclined  plane.*  A  fall  of  3  feet  per 
mile  will  make  a  mountain  torrent.  The  current  moves 
more  swiftly  at  the  centre  than  near  the  shores  or  bottom  of 
a  channel,  since  there  is  less  friction. 

4.  "Water-wheels  are  machines  for  using  the  force  of 
falling  water.  By  bands  or  cog-wheels  the  motion  of  the 
wheel  is  conducted  from  the  axle  into  the  mill,  f 

The  OVERSHOT-WHEEL  has  on  its  circumference  a  series  of 
buckets  which  receive  the  water  flow- 
ing from  a  sluice,  C.  These  hold  the 
water  as  they  descend  on  one  side, 
and  empty  it  as  they  come  up  on  the 
other.  Overshot-wheels  are  valuable 
where  a  great  fall  can  be  secured, 
since  they  require  but  little  water. 
If  P  denotes  the  weight  of  the  water 
and  d  the  distance  it  falls,  then  the 
total  work  =  Pd.  Of  this  amount 
75  per  cent,  can  be  made  available. 

The  UNDERSHOT- WHEEL  has  projecting  boards  or  floats, 
which  receive  the  force  of  the  current.  It  is  of  use  where 
there  is  little  fall  and  a  large  quantity  of  water.  It  utilizes 
about  25  per  cent,  of  the  force. 

FIG.  86.  FIG.  87. 


*  "  The  fall  of  800  ft.  would  theoretically  give  a  velocity  of  more  than  150  miles 
per  hour.  This  is  reduced  by  friction  to  about  3  miles." 

t  The  principle  is  that  of  a  lever  with  the  P  acting  on  the  short  arm.  In  this  way 
the  movement  of  the  slow  creaking  axle  reappears  in  the  swiftly  buzzing  saw  oi 
flying  spindle  (p.  73). 


100 


PRESSURE    OP    LIQUIDS    AND    GASES, 


The  BREAST-WHEEL  (Fig.  87)  is  a  medium  between  the 
two  kinds  already  named. 

The  TURBINE- WHEEL  is  placed  horizontally  and  immersed 
in  the  water.  In  Fig.  88,  0  is  the  dam  and  DA  the  spout 

by  which  the  water 
is  furnished.  E  is 
a  scroll-like  casing 

nn  I/A  -     H       A4-        encircling   the 

wheel>  and  °Pen  at 
K!I»         ±fa      centre    above 

and  below.  The 
axis  of  the  wheel  is 
the  cylinder/,  from 
which  radiate  plane- 
floats  against  which 
the  water  strikes. 
To  confine  the  wa- 
ter at  the  top  and 
the  bottom  is  a 
circular  disk  at- 
tached to  the  cyl- 
inder and  the  floats. 
In  these  disks  are 
the  swells  project- 
ing above  and  below  for  discharging  the  water.  They  com- 
mence near  the  cylinder,  and  swelling  outward  scroll-shaped, 
form  openings  curved  toward  the  cylinder,  thus  emptying 
the  water  in  a  direction  opposite  to  that  in  which  it  enters 
the  wheel.  This  form  utilizes  as  high  as  90  per  cent,  of 
the  force.  F  is  a  band- wheel  which  conducts  the  power  to 
the  machinery. 

The  principle  of  the  unbalanced  pressure  of  a  column 
of  water  may  also  be  employed.  It  is  illustrated  in  the  old- 
fashioned  Barker's  Mill  or  Reaction- Wheel.*  This  consists 
of  an  upright  cylinder  with  horizontal  arms,  on  the  opposite 


*  Revolving  fire-works  and  the  whirl-i-gig,  used  for  watering  lawns  and  as  an 
ornament  in  foiuitains,  are  constructed  on  the  same  principle.— An  ingenious  pupil 


HYDRAULICS. 


101 


FIG.  89. 


sides  of  which  are  small  apertures.  It  rests  in  a  socket,  so 
as  to  revolve  freely.  Water  is  supplied  from  a  tank  above. 
If  the  openings  in  the  arms  are  closed,  when  the  cylinder  is 
filled  with  water  the  pressure  is  equal  in  all  directions  and 
the  machine  is  at  rest.  If  now  we 
open  an  aperture,  the  pressure  is  re- 
lieved on  that  side,  and  the  arm  flies 
back  from  the  unbalanced  pressure  of 
the  column  of  water  above. 

5.  "Waves  are   produced  by  the 
friction  of  the  wind  against  the  sur- 
face of  the  water.     The  wind  raises 
the   particles   of  water   and    gravity 
draws  them  back  again.     They  thus 
vibrate  up  and  down,  but  do  not  ad- 
vance.*   The  forward  movement  of 
the  wave  is  an  illusion.     The  form  of 
the  wave  progresses,  but  not  the  water 
of  which  it  is  composed,  any  more 
than  the  thread  of  the  screw  which  we 
turn  in  our  hand,  or  the  undulations 
of  a  rope  or  carpet  which 
is  shaken,  or  the   stalks 
of  grain  which  bend  in 
billows  as  the  wind  sweeps 
over  them. 

The  corresponding 
parts  of  different  waves 
are  said  to  be  like  phases. 
The  distance  between  two  like  phases,  or  between  the  crests 

can  easily  construct  a  Reaction- wheel  of  straws  or  quills,  pouring  the  water  into  the 
upright  tube  by  means  of  a  pitcher  or  admitting.it  slowly  through  a  siphon  from  a 
pail  of  water  placed  on  a  table  above. 

*  Near  the  shore  the  oscillations  are  shorter,  and  the  waves  unbalanced  by  the  deep 
water,  are  forced  forward  till  the  lower  part  of  each  one  is  checked  by  the  friction  on 
the  sandy  beach,  the  front  becomes  well-nigh  vertical,  and  the  upper  part  curls  over 
and  falls  beyond.  The  size  of  "mountain  billows "  has  been  exaggerated.  Along 
the  coast  they  may  reach  90  feet,  but  in  the  open  sea  the  highest  wave,  from  the 
deepest  "  trough  "  to  the  very  topmost  "  crest,"  rarely  measures  over  30  feet. 


102 


PRESSURE    OF    LIQUIDS    AND    GASES. 


of  two  succeeding  waves,  is  called  a  wave-length.  Opposite 
phases  are  those  parts  which  are  vibrating  in  different 
directions,  as  the  point  midway  in  the  front  of  one  wave 
and  another  midway  in  the  rear  of  the  next  wave.  A  tide- 
wave  may  he  setting  steadily  toward  the  west ;  waves  from 
distant  storms  may  he  moving  upon  this ;  and  above  all, 
ripples  from  the  breeze  then  blowing  may  diversify  the 

FIG.  90. 


surface.*  These  different  systems  will  be  distinct,  yet  the 
joint  effect  may  be  very  peculiar.  If  any  two  systems  co- 
incide with  like  phases, — the  crest  of  one  meeting  the  crest 
of  the  other,  and  the  furrow  of  one  meeting  the  furrow  of 
the  other, — the  resulting  wave  will  have  a  height  equal  to 
the  sum  of  the  two.  If  any  two  coincide  with  opposite 

*  The  manner  in  which  different  waves  move  among  and  upon  one  another,  is 
seen  by  dropping  a  handful  of  stones  in  water  and  watching  the  waves  as  they  circle 
out  from  the  various  centres  in  ever-widening  curves.  In  Fig.  90  is  shown  the 
beautiful  appearance  these  waves  present  when  reflected  from  the  sides  of  a  vessel. 


PNEUMATICS. 


103 


phases, — the  hollow  of  one  striking  the  crest  of  another, — 
the  height  will  be  the  difference  of  the  two.  Thus,  if  in  two 
systems  having  the  same  wave-length  and  height,  one  is 
exactly  half  a  length  behind  the  other,  they  will  destroy  each 
other.*  This  is  termed  the  interference  of  waves. 

PRACTICAL  QUESTIONS.— 1.  How  much  more  water  can  be  drawn  from  a 
faucet  8  feet  than  from  one  4  feet  below  the  surface  of  the  water  in  a  cistern  5^~2.  How 
much  water  will  be  discharged  per  second  from  a  short  pipe  having  a  diameter  of 
4  inches  and  a  depth  of  48  feet  below  the  surface  of  the  water?  3.  When  we  pour 
molasses  from  a  jug,  why  is  the  stream  so  much  larger  near  the  nozzle  than  at  some 
distance  from  it?  4.  Ought  a  faucet  to  extend  into  a  barrel  beyond  the  staves? 
5.  What  would  be  the  effect  if  both  openings  in  one  of  the  arms  of  Barker's  Mill 
were  on  the  same  side  ? 


III.     PNEUMATICS. 

Pneumatics  treats  of  the  general  properties  and  the 
pressure  of  gases.  Since  the  molecules  move  among  one 
another  more  freely  even  than  those  of  liquids,  the  conclu- 
sions which  we  have  reached  with  regard  to  transmission  of 
pressure,  buoyancy  and  specific  gravity  apply  also  to  gases. 
As  air  is  the  most  abundant  gas,  it  is  taken  as  the  type  of 
the  class,  as  water  is  of  liquids. 

1 .  The  Air-pump  is  shown 
in  its  essential  features  in  Fig. 
91.  A  is  a  glass  receiver  stand- 
ing on  an  oiled  pump-plate. 
The  tube  D,  connecting  the 
receiver  with  the  cylinder,  is 
closed  by  the  valve  E  opening 
upward.  There  is  a  second 


*  In  the  port  of  Bateha  the  tidal  wave  comes  up  by  two  distinct  channels  so  un- 
equal in  length  that  their  time  of  arrival  varies  by  six  hours.  Consequently  when 
the  crest  of  high  water  reaches  the  harbor  by  one  channel,  it  meets  the  low  water 
returning  by  the  other,  and  when  these  opposite  phases  are  equal,  they  neutralize 
each  other  so  that  at  particular  seasons  there  is  no  tide  in  the  port,  and  at  other  times 
there  is  but  one  tide  per  day,  and  that  equal  to  the  differed  between  the  ordinary 
morning  and  evening  tide.— Lloyd's  Wave  Theory. 


104  PRESSURE    OF    LIQUIDS    AND    GASES. 

valve,  P,  in  the  piston,  also  opening  upward.  Suppose  the 
piston  is  at  the  bottom  and  both  valves  shut.  Let  it  now 
be  raised,  and  a  vacuum  will  be  produced  in  the  cylinder ; 
the  expansive  force  of  the  atmosphere  in  the  receiver  will 
open  the  valve  E  and  drive  the  air  through  to  fill  this  empty 
space.  When  the  piston  descends,  the  valve  E  will  close, 
while  the  valve  P  will  open,  and  the  air  will  pass  up  above 
the  piston.  On  elevating  the  piston  a  second  time,  this  air 
is  removed  from  the  cylinder,  while  the  air  from  the  re- 
ceiver passes  through  as  before.  At  each 
stroke  a  portion  of  the  atmosphere  is  drawn 
off;  but  the  expansive  force  becomes  less 
and  less,  until  finally  it  is  insufficient  to 
lift  the  valves.  For  this  reason  a  perfect 
vacuum  cannot  be  obtained. 

2.  The  Condenser,  in  construction,  is 
the  reverse  of  the  air-pump.     It  is  used  to 
force  into  a  vessel  an  increased  quantity 
of  air.* 

3.  Properties  of  Air. — (1.)  WEIGHT.— 
Exhaust  the  air  from  a  flask  which  holds 
100  cubic  inches,  and  then  balance  it.     On 

turning  the  stop-cock,  the  air  will  rush  in  with  a  whizzing 
noise  and  the  flask  descend.  It  will  require  31  grains  to 
restore  the  equipoise,  f 

*  The  practical  applications  of  this  pump  are  numerous.  The  soda  manufacturer 
uses  it  to  condense  carbonic  acid  in  soda-water  reservoirs.— The  engineer  employs  it 
in  laying  the  foundations  of  bridges.  Large  tubes  or  caissons  are  lowered  to  the 
bed  of  the  stream,  and  air  being  forced  in,  drives  out  the  water.  The  workmen  are 
let  into  the  caissons  by  a  sort  of  trap,  and  work  in  this  condensed  atmosphere  with 
comfort. — Pneumatic  despatch -tubes  contain  a  kind  of  train  holding  the  mail, 
and  back  of  this  a  piston  fitting  the  tube.  Air  is  forced  in  behind  the  piston  or  ex- 
hausted before  it,  and  so  the  train  is  driven  through  the  tube  at  a  high  speed.— In  the 
Westinghouse  air-brake,  condensed  air  is  forced  along  a  tube  running  underneath 
the  cars,  and  by  its  elastic  force  drives  the  brakes  against  the  wheel. 

t  Hermetically  close  one  end  of  a  piece  of  iron  gas-pipe,  and  fit  a  stop-cock  to  the 
other.  With  a  condenser  crowd  into  the  tube  several  atmospheres.  Weigh  the  tube 
in  a  grocer's  balance.  Turn  the  stop-cock  and  let  the  air  escape.  Then  the  beam 
will  rise.  The  amount  of  the  weights  required  to  be  added  to  restore  the  equilibrium 
«r ill  show  the  weight  of  the  condensed  air. 


PNEUMATICS. 


105 


FIG.  93. 


(2.)  ELASTICITY  is  shown  in  a  pop-gun.     We  compress 

the  atmosphere  in  the  barrel  until  the  elastic  force  drives 

out  the  stopper  with  a  loud  report.     As  we  crowd  down 

the  piston  we  feel  the  elasticity  of  the  air  yielding  to  our 

strength,  like  a  bent  spring. — The  bottle-imps,  or  Cartesian 

divers,  illustrate  the  same  property.     Pig.  93  represents  a 

simple  form  of  this  apparatus.     The  cover 

of  a  fruit-jar  is  fitted  with  a  tin  tube,  which 

is  inserted  in  a  syringe-bulb.     The  jar  is 

filled  with  water  and    the    diver  placed 

within.     This  is  a  hollow  image  of  glass, 

having  a  small  opening  at  the  end  of  the 

curved  tail.    If  we  squeeze  the  bulb,  the  air* 

will  be  forced  into  the  jar  and  the  water 

will  transmit  the  pressure  to  the  air  in  the 

image.     This  being  compressed,  more  water 

will    enter,   and    the  diver  will  descend. 

On  relaxing  the  grasp  of  the  hand  on  the 

bulb,  the  air  will  return  into  it,  the  air  in 

the  image  will  expand,  by  its  elastic  force 

driving  out  the  water,  and  the  diver,  thus 

lightened  of  his  ballast,  will  ascend.     The 

nearer  the  image  is  to  the  bottom,  the  less 

force  will  be  required  to  move  it.     With  a 

little  care  it  can  be  made  to  respond  to  the  slightest  pres- 
sure, and  will  rise  and  fall  as  if  instinct 
with  life.* 

(3.)  EXPANSIBILITY. — Let  a  well-dried 
bladder  be  partly  filled  with  air  and 
tightly  closed.  Place  it  under  the  re- 
ceiver and  exhaust  the  air.  The  air  in 
the  bladder  expanding  will  burst  it  into 
shreds. 

Take  two  bottles  partly  filled  with 

*  This  experiment  shows  also  the  buoyant  force  of  liquids,  their  transmission  of 
pressure  in  every  direction,  the  increase  of  the  pressure  in  proportion  to  the  depth, 
and  the  principle  of  Barker's  Mill.  (See  note, 


FIG.  94. 


106 


PRESSURE    OP    LIQUIDS    AND    GASES. 


FIG.  95. 


FIG.  96. 


colored  water.  Let  a  bent  tube  be  inserted  tightly  in  A  and 
loosely  in  B.  Place  this  apparatus 
under  the  receiver  and  exhaust  the  air. 
The  expansive  force  of  the  air  in  A  will 
drive  the  water  over  into  B.  On  re- 
admitting the  air  into  the  receiver,  the 
pressure  will  return  the  water  into  A. 
It  may  thus  be  driven  from  bottle  to 
bottle  at  pleasure.* 

Hero* s  fountain  acts  on  the  same  principle, 
as  may  be  seen  by  an  examination  of  Fig.  96. 
Having  removed  the  jet-tube,  the  upper  globe 
is  partly  filled  with  water.  The  tube  being 
then  replaced,  water  is  poured  into  the  basin 
on  top.  The  liquid  runs  down  the  pipe 
at  the  right,  into  the  lower  globe.  The  air 
in  that  globe  is  driven 
up  the  tube  at  the 
left,  into  the  upper 
globe,  and  by  its  elas- 
ticity forces  the  wa- 
ter there  out  through 
the  jet-tube,  forming 
a  tiny  fountain. 

4.  Pressure  of  the 
Air.— (1.)  THE  PROOF. 
— If  we  cover  a  hand-glass  with  one 
hand,  as  in  Fig.   97,  on  exhausting 
the  air  we  shall  find  the  pressure  painful,  f     Tie  over  one 


FIG.  97. 


*  Prick  a  hole  in  the  small  end  of  an  egg  and  place  the  egg  with  the  big  end  np 
in  a  wine-glass.  On  exhausting  the  receiver,  the  bubble  of  air  in  the  upper  pert  of 
the  egg  will  drive  the  contents  down  into  the  glass,  and  on  admitting  the  air  they 
will  be  forced  back  again. 

•t  The  exhaustion  of  the  air  does  not  produce  the  pressure  on  the  hand ;  it  simply 
reveals  it.  The  average  pressure  on  each  person  is  16  tons.  It  is  equal,  however, 
on  all  parts  of  the  body  and  is  counteracted  by  the  air  within.  Hence  we  never 
notice  it  Persons  who  go  up  high  mountains  or  g«  down  in  diving-bells  feel  the 
change  in  the  pressure. 


PNEUMATICS. 


107 


When  dry,  exhaust 


FIG.  98. 


FIG.  99. 


end  of  the  glass  a  piece  of  wet  bladder, 
the  air,  and  the  mem- 
brane will  burst  with  a 
sharp  report.* 

The  Magdeburg  Hemi- 
spheres are  named  from 
the  city  in  which  Guer- 
icke,  their  inventor,  re- 
sided. They  consist  of 

two  small  brass  hemispheres,  which  fit  closely  together,  but 
may  be  separated  at  pleasure.  If,  how- 
ever, the  air  be  exhausted  from  within, 
several  persons  will  be  required  to  pull 
them  apart,  f  In 
whatever  position 
the  hemispheres 
are  held,  the  pres- 
sure is  the  same. 
(2. )  UPWARD 
PRESSURE.  —  Fill 

a  tumbler  with  water,  and  then  lay 
a  sheet  of  paper  over  the  top. 
Quickly  invert  the  glass,  and  the 
water  will  be  supported  by  the  up- 
ward pressure  of  the  air. — "Within 
the  glass  cylinder,  Fig.  100,  is  a 
piston  working  air-tight.  Connect 
C  with  the  pump  by  a  rubber  tube 


*  To  show  the  crushing  force  of  the  atmosphere,  take  a  tin  cylinder  15  inches 
long  and  4  inches  in  diameter.  Fit  one  end  with  a  stop-cock  or  merely  leave  a  hole 
for  the  exit  of  the  steam.  Put  in  a  little  water  and  boil.  When  the  air  is  entirely 
driven  out,  turn  the  stop-cock  or  close  the  opening  with  a  bit  of  solder.  Pour  cold 
water  over  the  outside  to  condense  the  steam,  when  the  cylinder  will  collapse  as  if 
struck  by  a  heavy  blow. 

t  In  the  Museum  at  Berlin  the  hemispheres  used  by  Guericke  in  his  experiments 
are  preserved.  They  are  of  copper,  and,  by  the  author's  measurements,  22  inches 
interior  diameter  with  a  flange  an  inch  wide,  making  the  entire  diameter  2  feet. 
Accompanying  is  a  Latin  book  by  the  burgomaster  describing  numerous  pneumatic 
experiments  which  he  had  performed,  and  containing  a  wood-cut  representing  three 
spans  of  horses  oil  each  side  trying  to  separate  the  hemispheres. 


108 


PRESSURE    OF    LIQUIDS    AND    GASES. 


and  exhaust  the  air.     The  weight  will  leap  up  as  if  caught 
by  a  spring. 

(3.)  BUOYANT  FORCE  OF  THE  AIR. — The  law  of  Archi- 
medes (p.  93)  holds  true  in  gases.  Smoke  and  other  light 
substances  float  in  the  air,  as  wood  does  in  water,  because 
they  are  lighter  and  are  buoyed  with  a  force  equal  to  the 
weight  of  the  air  they  displace.  A  hollow  sphere  of  copper, 
Fig.  101,  is  balanced  in  the  air  by  a  solid  lead  weight,  but 
it  instantly  falls  on  being  placed  under  the  receiver  and  the 

FIG.  101.  FIG.  102. 


air  exhausted.  This  shows  that  its 
weight  was  partly  sustained  by  the 
buoyant  force  of  the  air. 

(4.)  THE  PRESSURE  OF  THE  AIR 
SUSTAINS  A  COLUMN  OF  MERCURY  30 
INCHES  HIGH,  OF  WATER  34  FEET 
HIGH,  AND  IS  15  LBS.  PER  SQUARE 
INCH.  Take  a  strong  glass  tube 
about  three  feet  in  length,  and  j| 
tie  over  one  end  a  piece  of  wet 
bladder.  When  dry,  fill  the  tube  with  mercury,  and  invert 
it  in  a  cup  of  the  same  liquid.  The  mercury  will  sink  to  a 
height  of  about  30  in.  If  the  area  across  the  tube  be  1  sq. 
in.,  the  metal  will  weigh  nearly  15  Ibs.  The  weight  of  the 
column  of  mercury  is  equal  to  the  downward  pressure  on 


PNEUMATICS. 


109 


each  square  inch  of  the  surface  of  the  mercury  in  the  cup. 
Hence  we  conclude  that  the 
pressure  of  the  atmosphere  FlG-  m 
is  nearly  15  Ibs.  per  sq.  in., 
and  will  balance  a  column 
of  mercury  30  inches  high. 
As  water  is  13  J  times  light- 
er than  mercury,  the  same 
pressure  would  balance  a 
column  of  that  liquid  13£ 
times  higher,  or  33}  feet.* 

(5.)  PEESSUEE  OF  THE 
AIE  VAEiES.f  Changes  of 
temperature,  moisture,  etc., 
constantly  vary  the  pressure 
of  the  air,  and  change  the 
height  of  the  column  of 
liquid  it  can  support.  The 
pressure  of  the  air  also 
increases  with  the  depth. 
Hence,  in  a  valley  its 
pressure  is  greater  than  on 
a  mountain.  The  figures 
given  in  the  last  paragraph 
apply  only  to  the  level  of 
the  sea  and  a  temperature 
of  60°  F.  They  are  the 
standards  for  reference. 

(6.)  MAEIOTTE'S  LAW. — Fig.  103  represents  a  long,  bent  I 


*  Pour  OB  the  mercury  in  the  cup  (Fig.  102)  a  little  water  colored  with  red  ink. 
Then  raise  the  end  of  the  tube  above  the  surface  of  the  metal,  but  not  above  that  of 
the  water  which  will  rise  in  the  tube,  the  mercury  passing  down  in  beautifully-beaded 
globules.  The  mercurial  column  is  only  30  inches  high,  while  the  water  will  fill  the 
tube.  Finish  the  experiment  by  puncturing  the  bladder  with,  a  pin,  when  the  water 
will  instantly  fall  to  the  cup  below. 

t  We  live  at  the  bottom  of  an  ae'rial  ocean  whose  depth  is  many  times  that  of  the 
deepest  sea.  Its  invisible  tides  surge  round  us  on  every  side.  More  restless  than 
the  sea,  its  waves  beat  to  and  fro,  and  never  know  a  calm. 

$  By  cautiously  inclining  the  apparatus,  when  a  little  air  will  escape,  and  adding 
more  mercury  if  needed,  the  liquid  can  be  made  to  stand  at  zero  in  both  arms. 


110  PRESSURE    OF    LIQUIDS    AND    GASES. 

glass  tube  with  the  end  of  the  short  arm  closed. 
Pour  mercury  into  the  long  arm  until  it  rises  to 
the  point  marked  zero.  It  stands  at  the  same 
height  in  both  arms,  and  there  is  an  equilibrium. 
The  air  presses  on  the  mercury  in  the  long  arm 
with  a  force  equal  to  a  column  of  mercury  30 
inches  high,  and  the  elastic  force  of  the  air  con- 
fined in  the  short  arm  is  equal  to  the  same  amount. 
Now  pour  additional  mercury  into  the  long  arm 
until  it  stands  at  30  inches  above  that  in  the  short 
arm  (Fig.  104),  and  the  pressure  is  doubled.  In 
the  short  arm,  the  air  is  condensed  to  one-half  its 
former  dimensions,  and  the  expansive  force  is  also 
doubled.*  We  therefore  conclude  that  the  elas- 
ticity of  a  gas  increases,  and  the  volume  diminishes 
in  proportion  to  the  pressure  upon  it. 

(7.)  The  BAROMETER  is  an  instrument  for 
measuring  the  pressure  of  the  air.  It  consists 
essentially  of  the  tube  and  cup  of  mercury  in 
Fig.  102.  A  scale  is  attached  for  convenience  of 
reference.  The  barometer  is  used  (a)  to  indicate 
the  weather,  and  (£)  to  measure  the  height  of 
mountains. 

It  does  not   directly  foretell   the  weather.     It 
simply  shows  the  varying  pressure  of  the  air,  from 
which  we  must  draw  our  conclusions.      A  con- 
tinued rise  of  the  mercury  indicates  fair  weather,  and  a  con- 
tinued fall,  foul  weather,  f     Since  the  pressure  diminishes 
above  the  level  of  the  sea,  the  observer  ascertains  the  fall  of 


*  The  force  with  which  the  flying  molecules  of  air  (note,  p.  50)  beat  against  the 
walls  of  any  confining  vessel  will  increase  with  the  diminution  of  the  space  through 
which  they  can  pass.  If  we  give  them  only  half  the  distance  to  fly  through,  they  will 
strike  twice  as  often  and  exert  twice  the  pressure. 

t  Mercury  is  used  for  filling  tho  barometer  because  of  its  weight  and  low  freezing- 
point.  It  is  said  that  the  first  barometer  was  filled  with  water.  The  inventor,  Otto 
von  Quericke,  erected  a  tall  tube  reaching  from  a  cistern  in  the  cellar  up  through  the 
roof  of  his  house.  A  wooden  image  was  placed  within  the  tube,  floating  upon  the 
water.  On  fine  days,  this  novel  weather-prophet  would  rise  above  the  roof-top  and 
peep  out  upon  the  queer  old  gables  of  that  ancient  city,  while  in  foul  weather  he 


PNEUMATICS. 


111 


the  mercury  in  the  barometer,  and  the  temperature  by  the 
thermometer ;  and  then,  by  reference  to  tables,  determines 
the  height. 

5.  Pumps. — (1.)  The  LIFTING-PUMP  contains  two  valves 
opening  upward — one,  a,  at  the  top  of  the  suction-pipe,  B ; 
the  other,  c,  in  the  piston.  Suppose  the  handle  to  be  raised, 


FIG.  106. 


FIG.  107. 


FIG.  108. 


the  piston  at  the  bottom  of  the  cylinder  and  both  valves 
closed.  Now  depress  the  pump-handle  and  elevate  the  pis- 
ton. This  will  produce  a  partial  vacuum  in  the  suction- 
pipe.  The  pressure  of  the  air  on  the  surface  of  the  water 
below  will  force  the  water  up  the  pipe,  open  the  valve,  and 


would  retire  to  the  protection  of  the  garret.  The  accuracy  of  these  movements 
attracted  the  attention  of  the  neighbors.  Finally,  becoming  suspicious  of  Otto's 
piety,  they  accused  him  of  being  in  league  with  the  devil.  So  the  offending  philoso- 
pher relieved  this  wicked  wooden  man  from  longer  dancing  attendance  upon  the 
weather,  and  the  staid  old  city  was  once  more  at  peace. 


PRESSURE    OF    LIQUIDS    AKD    GASES. 


partly  fill  the  chamber.  Let  the  pump-handle  be  ele- 
vated again,  and  the  piston  depressed.  The  valve  a  will 
then  close,  the  valve  c  will  open  and  the  water  will  rise 
above  the  piston  (Fig.  107).  When  the  pump-handle  is 
lowered  the  second  time  and  the  piston  elevated,  the  water 
is  lifted  up  to  the  spout,  whence  it  flows  out ;  while  at  the 
same  time  the  lower  valve  opens  and  the  water  is  forced  up 
from  below  by  the  pressure  of  the  air  (Fig.  108).* 


FIG.  109. 


The  FORCE-PUMP  has  no  valve  in  the  piston.  The 
water  rises  above  the  lower  valve  as  in 
the  lifting-pump.  When  the  piston  de- 
scends, the  pressure  opens  the  valve  and 
forces  the  water  up  the  pipe  D.  This 
pipe  may  be  made  of  any  length,  and  thus 
the  water  driven  to  any  height. 

(3.)  The  FIRE-E^GI^E  consists  of  two 
force-pumps  with  an  air-chamber.  The 
water  is  driven  by  the  pistons  m,  n,  alter- 
nately, into  the  chamber  R,  whence  the 
air,  by  its  expansive  force,  throws  it  out 
in  a  continuous  stream  through  the  hose- 
pipe attached  at  Z  (Fig.  110). 

6.  The  Siphon  is  a  U-shaped  tube, 
having  one  arm  longer  than  the  other. 
Insert  the  short  arm  in  the  water,  and 
then  applying  the  mouth  to  the  long 
arm,  exhaust  the  air.  The  water  will  flow 
from  the  long  arm  until  the  end  of  the 
short  arm  is  uncovered,  f 

*  If  the  valves  and  piston  were  fitted  air-tight,  the  water  could  be  raised  34  feet 
(more  exactly  13^  times  the  height  of  the  barometric  column)  to  the  lower  valve,  but 
owing  to  various  imperfections  it  commonly  reaches  about  28  feet.  For  a  similar 
reason  we  sometimes  find  a  dozen  strokes  necessary  to  "bring  water." 

t  An  instructive  experiment  may  be  given  if  we  allow  the  water  to  run  from  one 
tumbler  into  another  until  just  before  the  flow  ceases ;  then  quickly  elevate  the  glass 
containing  the  long  arm,  carefully  keeping  both  ends  of  the  siphon  under  the  water, 
when  the  flow  will  set  back  to  the  first  tumbler.  Thus  we  may  alternate  until  we 


PNEUMATICS. 


113 


Flo.  110. 


Fio.  111. 


THEORY    OF    THE 
SIPHON. — The    pres- 
sure of  the  air  at  b 
holds  up  the  column 
of  water  a  b,  and  the 
upward    pressure    is 
the  weight  of  the  air 
less  the  weight  of  the 
column  of  water  a 
The  upward  pressure 
at  d  is  the 
weight     of 
the    air 
minus    the 
weight     of 
the  column 


see  that  the  water  flows  to  the  lower  level,  and  ceases  whenever  it  reaches  the  same 
level  in  both  glasses.  It  will  add  to  the  beauty  of  this  as  well  as  of  many  other 
experiments,  to  color  the  water  with  a  few  scales  of  magenta,  or  with  red  ink. 


114 


PEESSUEE    OF    LIQUIDS    AND    GASES. 


of  water  c  d.     Now  c  d  is  less  than  a  b,  and  the  water  in 

the  tube   is   driven  toward   the 
?IG-118-  longer  arm  by  a  force  equal  to 

the  difference  in  the  weight  of 
the  two  arms. 

7.  The  Pneumatic  Inkstand 
can  be  filled  only  when  tipped  so 
that  the  nozzle  is  at  the  top. 
The  pressure  of  the  air  will  retain 
the  ink  when  the  stand  is  placed 

upright.     When  used  below  o,  a  bubble  of  air  passes  in, 

forcing  the  ink  into  the  nozzle. 

8.  The  Hydraulic  Ram  is  a  machine  for  raising  water 
where  there  is  a  slight  fall.     The  water  enters  through  the 

FIG.  113. 


pipe  A,  fills  the  reservoir  B,  and  lifts  the  valve  D.  As  that 
closes,  the  shock  raises  the  valve  E  and  drives  the  water 
into  the  air-chamber  G.  D  falls  again  as  soon  as  an  equi- 
librium is  restored.  A  second  shock  follows,  and  more 
water  is  thrown  into  G.  When  the  air  in  G  is  sufficiently 
condensed,  its  elastic  force  drives  the  water  through  the 
pipe  H. 

9.    The  Atomizer  is  used  to  turn  a  liquid  into  spray. 


PNEUMATICS. 


115 


FIG.  114. 


The  blast  of  air  driven  from  the  rubber  bulb,  as  it  passes 
over  the  end  of  the  upright 
tube,  sweeps  along  the  neigh- 
boring molecules  of  air  and 
produces  a  partial  vacuum  in 
the  tube.*  The  pressure  of 
the  air  in  the  bottle  drives 
the  liquid  up  the  tube,  and 
at  the  mouth  the  blast  of  air 
carries  it  off  in  fine  drops. 

The  action  of  a  current  of 
air  in  dragging  along  with  it 
the  adjacent  still  atmosphere 
and  so  tending  to  produce  a 
vacuum,  is  shown  by  the  ap- 
paratus represented  in  Fig.  115. 

Tio.  118. 


A  globe,  a,  is  connected 


*  In  locomotives,  this  principle  of  the  adhesion  of  gases  to  gases  is  applied  to 
produce  a  draft.  The  waste  steam  is  thrown  into  the  smoke-pipe,  and  this  current 
sweeps  off  the  smoke  from  the  fire,  while  the  pressure  of  the  atmosphere  outside 
forces  the  air  through  the  furnace  and  increases  the  combustion.— A  familiar  illus- 
tration may  be  devised  by  taking  two  discs  of  cardboard,  the  lower  one  fitted  with  a 
quill,  and  the  upper  one  merely  kept  from  sliding  off  by  a  pin  thrust  through  it  and 
extending  into  the  quill.  The  more  forcibly  air  is  driven  through  the  quill  against 
the  upper  disc,  the  more  firmly  it  will  be  held  to  its  place.  See  article  u  Ball  Para- 
dox," in  Popular  Science  Monthly,  April,  1877.— Faraday  used  to  illustrate  the  prin- 
ciple thus :  Hold  the  hand  out  flat  with  the  fingers  extended  and  pressed  together. 
Place  underneath  a  piece  of  paper  two  inches  square.  Blow  through  the  opening 
between  the  index  and  the  middle  finger,  and  so  long  as  the  current  is  passing  the 
paper  will  not  fall. 


116  PRESSURE    OF    LIQUIDS    AND    GASES. 

with  a  horizontal  tube,  c,  containing  colored  water.  Close 
the  opening  d  with  the  finger,  and  with  the  mouth  at  b 
draw  the  air  out  of  the  globe.  A  slight  rarefaction  will 
cause  the  liquid,  by  the  pressure  of  the  air  at  the  opening  /, 
to  be  forced  into  «.  Now,  if,  instead  of  drawing  the  air 
out  at  b,  a  jet  of  air  be  forced  through  the  tube  and  out  at 
d,  the  same  effect  will  be  produced. 

1C.  Height  of  the  Air. — Three  opposing  forces  act 
upon  the  air,  viz. :  gravity,  which  binds  it  to  the  earth,  and 
the  centrifugal  and  repellent  forces,  which  tend  to  hurl  it 
into  space.  There  must  be  a  point  where  these  balance.  At 
the  height  of  3.4  miles  the  mercury  in  the  barometer  stands 
at  15  inches,  indicating  that  half  the  atmosphere  is  within 
about  3J  miles  of  the  earth's  surface.  The  height  of  the 
atmosphere  is  variously  stated  at  from  50  to  500  miles. 

PRACTICAL  QUESTIONS.— 1.  Why  must  we  make  two  openings  in  a  barrel  of 
cider  when  we  tap  it  ?  2.  What  is  the  weight  of  10  cubic  feet  of  air  ?  3.  What  is 
the  pressure  of  the  air  on  1  square  rod  of  land  ?  4.  What  is  the  pressure  on  a  pair 
of  Magdeburg  hemispheres  4  inches  in  diameter  ?  5.  How  high  a  column  of  water 
can  the  air  sustain  when  the  barometric  column  stands  at  28  inches  ?  6.  If  we  should 
add  a  pressure  of  two  atmospheres  (30  Ibs.  to  the  square  inch),  what  would  be  the 
volume  of  100  cubic  inches  of  common  air  ?  7.  If,  while  the  water  is  running  through 
the  siphon,  we  quickly  lift  the  long  arm,  what  is  the  effect  on  the  water  in  the  siphon  ? 
If  we  lift  the  entire  siphon  ?  8.  When  the  mercury  stands  at  29£  inches  in  the 
barometer,  how  high  above  the  surface  of  the  water  can  we  place  the  lower  pump- 
valve  ?  9.  Can  we  raise  water  to  a  higher  level  by  means  of  a  siphon  ?  10.  If  the 
air  in  the  chamber  of  a  fire  engine  be  condensed  to  TV  its  former  bulk,  what  will  be 
the  pressure  due  to  the  expansive  force  of  the  air  on  every  square  inch  of  the  air- 
chamber  ?  11.  What  causes  the  bubbles  to  rise  to  the  surface  when  we  put  a  lump 
of  loaf-sugar  in  hot  tea  ?  12.  To  what  height  can  a  balloon  ascend  ?  What  weight 
can  it  lift  ?  13.  The  rise  and  fall  of  the  barometric  column  shows  that  the  air  is 
lighter  in  foul  and  heavier  in  fair  weather.  Why  is  this?  Ans.  Vapor  of  water  is 
only  half  as  heavy  as  dry  air.  When  there  is  a  large  quantity  present  in  the  atmos- 
phere, displacing  its  own  bulk  of  air,  the  weight  of  the  atmosphere  will  be  cor- 
respondingly diminished.  14.  When  smoke  ascends  in  a  straight  line  from  chimneys, 
is  it  a  proof  of  the  rarity  or  the  density  of. the  air?  15.  Explain  the  action  of  the 
common  leather-sucker.  16.  Did  you  ever  see  a  bottle  really  empty  ?  17.  Why  is  it 
so  tiresome  to  walk  in  miry  clay?  Ans.  Because  the  upward  pressure  of  the  air  is 
removed  from  our  feet.  18.  How  does  the  variation  in  the  pressure  of  the  air  affect 
those  who  ascend  lofty  mountains  ?  Who  descend  in  diving-bells  ?  19.  Explain  the 
theory  of  "  sucking  cider "  through  a  straw.  20.  Would  it  make  any  difference  in 
the  action  of  the  siphon  if  the  limbs  were  of  unequal  diameter?  21.  If  the  receiver 
of  an  air-pump  is  5  times  as  large  as  the  barrel,  how  many  strokes  of  the  piston  will 
be  needed  to  dimmish  the  air  nearly  one-half?  22.  What  would  be  the  effect 
of  making  a  small  hole  in  the  top  of  a  diving-bell  while  in  use  ?  23.  The  pressure 
of  the  atmosphere  being  1.03  kg.  per  sq.  cm.,  what  ia  the  amount  on  1  are  ?  On  10 
sq.  metres  f 


PNEUMATICS.  117 


SUM  MARY. 

Hydrostatics  treats  of  the  laws  of  equilibrium  in  liquids.  Pres- 
sure is  transmitted  by  liquids  equally  in  every  direction.  Water  thus 
becomes  a  "  mechanical  power/'  as  in  the  "  Hydraulic  Press."  Liquids 
acted  on  by  their  weight  only,  at  the  same  depth,  press  downward, 
upward,  and  sidewise  with  equal  force.  This  pressure  is  independent 
of  the  size  of  the  vessel,  but  increases  with  the  depth.  Wells,  springs, 
aqueducts,  fountains  and  the  water-supply  of  cities  illustrate  the  ten- 
dency of  water  to  seek  its  level.  The  ancients  understood  this  law, 
but  had  no  suitable  material  for  making  the  immense  pipes  needed  ; 
just  so  the  art  of  printing  waited  the  invention  of  paper.  Specific 
gravity,  or  the  relative  weights  of  the  same  bulk  of  different  sub- 
stances, is  found  by  comparing  them  with  the  weight  of  the  same 
bulk  of  water.  This  is  easily  done,  since,  according  to  the  law  of 
Archimedes,  a  body  immersed  in  water  is  buoyed  up  by  a  force  equal 
to  the  weight  of  the  water  displaced  ;  i.  c.,  it  loses  in  weight  an  amount 
equal  to  that  of  the  same  bulk  of  water.  Hence  spec.  grav.  = 

-   weightinair   m  A  floating  body  displaces  only  its 

weight  in  air  —  weight  in  water 

weight  of  liquid.     This   explains  the  buoyancy  of  a  ship,  why  a 
floating  log  is  partly  out  of  water,  and  many  similar  phenomena. 

Hydraulics  treats  of  moving  liquids.  The  laws  of  falling  bodies 
in  the  main  apply.  So  that  a  descending  jet  of  water  will  acquire  the 
same  velocity  that  a  stone  would  in  falling  to  the  ground  from  the  sur- 
face of  the  water;  and  an  ascending  jet  would  need  to  have  the  same 
velocity  in  order  to  reach  that  height.  The  quantity  of  water  dis- 
charged through  any  orifice  equals  the  area  of  the  opening  multiplied 
by  the  velocity  of  the  stream.  The  chief  resistance  to  the  motion  of  a 
liquid  is  the  friction  of  the  air  and  against  the  sides  of  the  pipe,  and, 
in  the  case  of  rivers,  against  the  banks  and  bottom  of  the  channel. 
The  force  of  falling  water  is  utilized  in  the  arts  by  means  of  water 
wheels.  There  are  four  kinds — overshot,  undershot,  breast,  and  tur- 
bine. The  principles  of  wave  motion,  so  essential  to  the  understanding 
of  sound,  light,  etc.,  are  easiest  studied  in  connection  with  water.  A 
stone  let  fall  into  a  quiet  pool  sets  in  motion  a  series  of  concentric 
waves,  whose  particles  merely  rise  and  fall,  while  the  movement 
passes  to  the  outermost  edge  of  the  water,  and  is  then  transmitted  to 
the  ground  beyond.  The  velocity  of  the  particles  is  much  less  than 
that  of  the  wave  itself.  A  handful  of  stones  acts  in  the  same  way, 


118  PRESSURE    OF    LIQUIDS    AND    GASES. 

but  sets  in  motion  many  series  of  waves.    Hence  arise  the  phenomena 
of  interference. 

'Pneumatics  treats  of  the  properties  and  the  laws  of  equilibrium 
of  gases.  The  air  being  composed  of  matter,  has  all  the  properties  we 
associate  with  matter,  as  weight,  indestructibility,  extension,  compres- 
sibility, etc.  In  addition,  it  is  remarkable  for  its  elasticity.*  The 
elasticity  of  the  air,  according  to  Mariotte's  (and  Boyle's)  law,  is 
inversely  proportional  to  its  volume,  and  that  is  inversely  proportional 
to  the  pressure  upon  the  air ;  both  heat  and  pressure  increasing  the 
elasticity  of  a  gas.  The  air,  like  other  fluids,  transmits  the  weight  ol 
its  own  particles,  as  well  as  any  outside  pressure,  equally  in  every 
direction  ;  hence  the  upward  pressure  or  buoyant  force  of  the  atmos- 
phere. A  balloon  rises  because  it  is  buoyed  up  by  a  force  equal  to  the 
weight  of  the  air  it  displaces.  It  floats  in  the  air  for  the  same  reason 
that  a  ship  floats  on  the  ocean.  When  smoke  falls  it  is  heavier,  and 
when  it  rises  it  is  lighter  than  the  surrounding  atmosphere.  The  air- 
pump  is  used  for  exhausting  the  air  from,  and  the  condenser  for  con- 
densing the  air  into,  a  receiver.  A  vacuum  in  which  there  remains 
only  -unj-tmnj-  of  the  atmosphere  can  be  obtained  by  means  of  Spren- 
gel's  air-pump,  which  acts  on  the  principle  of  the  adhesion  of  the  air 
to  a  column  of  falling  mercury.  The  average  weight  of  the  air  being 
15  Ibs.  to  the  square  inch,  equals  that  of  a  column  of  water  34  feet, 
and  of  mercury  30  inches  or  760  millimetres  high.  This  amount  varies 
incessantly  through  atmospheric  changes  caused  by  alterations  in  the 
wind,  heat  of  the  sun,  etc.  The  barometer  measures  the  weight  of  the 
atmosphere,  and  is  used  to  determine  the  height  of  mountains  and  the 
changes  of  the  weather.  The  action  of  the  siphon,  the  pneumatic  ink- 
stand,  and  of  the  different  kinds  of  pumps,  is  based  upon  the  pressure 
of  the  air. 

*  The  elasticity  of  the  air,  as  well  as  the  principles  explained  by  the  Cartesian 
diver,  Fig.  93,  may  be  illustrated  in  the  following  simple  manner:  Fill  with  water  a 
wide-mouth,  8-oz.  bottle,  and  also  a  tiny  vial,  such  as  is  used  by  homceopathists. 
Invert  the  vial  and  a  few  drops  of  water  will  run  out.  Now  put  it  inverted  into  the 
bottle,  and  if  it  does  not  sink  just  below  the  surface  and  there  float,  take  it  out  and 
add  or  remove  a  little  water,  as  may  be  needed.  When  this  result  is  reached,  cork 
the  bottle  so  that  the  cork  touches  the  water.  Any  pressure  on  the  cork  will  ..hen  be 
transmitted  to  the  air  in  tb.e  vial,  as  in  the  image  in  Fig.  93. 


HISTOEICAL    SKETCH.  119 


HISTORICAL    SKETCH. 

ffydrostatics  is  comparatively  a  modern  science.  The  Romans  had 
a  knowledge  of  the  fact  that  "  liquids  rise  to  the  level  of  their  source," 
but  they  had  no  means  of  making  iron  pipes  strong  enough  to  resist 
the  pressure.*  They  were  therefore  forced  to  carry  water  into  the 
imperial  city  by  means  of  enormous  aqueducts,  one  of  which  was  63 
miles  long,  and  was  supported  by  arches  100  feet  high.  The  ancient 
Egyptians  and  Chaldeans  were  probably  the  first  to  investigate  the 
most  obvious  laws  of  liquids  from  the  necessity  of  irrigating  their 
land.  Archimedes,  in  the  3d  century  B.  C.,  invented  a  kind  of  pump 
called  Archimedetf  Screw,  demonstrated  the  principle  of  equilibrium, 
known  now  as  "Archimedes'  Law  "  (p.  117),  and  found  out  the  method 
of  obtaining  the  specific  gravity  of  bodies.  The  discovery  of  the 
last  is  historical.  Hiero  of  Syracuse  suspected  that  a  gold  crown 
had  been  fraudulently  alloyed  with  silver.  He  accordingly  asked 
Archimedes  to  find  out  the  fact  without  injuring  the  workmanship  of 
the  crown.  One  day  going  into  a  bath-tub  full  of  water,  the  thought 
struck  the  philosopher  that  as  much  water  must  run  over  the  side  as 
was  equal  to  the  bulk  of  his  body.  Electrified  by  the  idea,  he  sprang 
out  and  ran  through  the  streets,  shouting:  "Eureka!"  (I  have 
found  it !) 

The  ancients  never  dreamed  of  associating  the  air  with  gross  mat- 
ter. To  them  it  was  the  spirit,  the  life,  the  breath.  Noticing  how 
the  atmosphere  rushes  in  to  fill  any  vacant  space,  the  followers  of 
Aristotle  explained  it  by  saying,  "  Nature  abhors  a  vacuum."  This 
principle  answered  the  purpose  of  philosophers  for  2,000  years.  In 
1640,  some  workmen  were  employed  by  the  Duke  of  Tuscany  to  dig 
a  deep  well  near  Florence.  They  found  to  their  surprise  that  the 
water  would  not  rise  in  the  pump  as  high  as  the  lower  valve.  More 
disgusted  with  nature  than  nature  was  with  the  vacuum  in  their 
pump,  they  applied  to  Galileo.  The  aged  philosopher  answered — 
half  in  jest,  we  hope,  certainly  he  was  half  in  earnest — "  Nature 
does  not  abhor  a  vacuum  beyond  34  feet."  His  pupil,  Torricelli,  how. 


*  The  ancient  engineers  sometimes  availed  themselves  of  this  principle.  Not  fat 
from  Rachel's  Tomb,  Jerusalem,  are  the  remains  of  a  conduit  once  used  for  supply- 
ing the  city  with  water.  The  valley  was  crossed  by  means  of  an  inverted  siphon. 
The  pipe  was  about  two  miles  long  and  fifteen  inches  in  diameter.  It  consisted 
of  perforated  blocks  of  stone,  ground  smooth  at  the  joints,  and  fastened  with  a  hard 
cement. 


120  PBESSUEE    OF    LIQUIDS    AND    GASES. 

ever,  discovered  the  secret.  He  reasoned  that  there  is  a  force  which  holds 
up  the  water,  and  as  mercury  is  13£  times  as  heavy  as  water,  it  would 
sustain  a  column  of  that  liquid  only  33  feet-r- 13^  =  30  inches  high. 
Trying  the  experiment  shown  in  Fig.  102,  he  verified  the  conclusior 
that  the  weight  of  the  air  is  the  unknown  force.  But  the  opinion  wag 
not  generally  received.  Pascal  next  reasoned  that  if  the  weight  of 
the  air  is  really  the  force,  then  at  the  summit  of  a  high  mountain  it 
Ls  weakened,  and  the  column  would  be  lower.  He  accordingly  car- 
ried his  apparatus  to  the  top  of  a  steeple,  and  finding  a  sHght  fall  in 
the  mercury,  he  asked  his  brother-in-law,  who  lived  near  Puy  de  Dome, 
a  mountain  in  Southern  France,  to  test  the  conclusion.  On  trial,  it  was 
found  that  the  mercury  fell  3  inches.  "A  result,"  wrote  Perrier, 
"which  ravished  us  with  admiration  and  astonishment."  Thus  was 
discovered  the  germ  of  our  modern  barometer,  and  the  dogma  of  the 
philosophers  soon  gave  place  to  the  law  of  gravitation  and  our  present 
views  concerning  the  atmosphere. 

Consult  Pepper's  "Cyclopaedic  Science";  Bert's  "Atmospheric 
Pressure  and  Life,"  in  Popular  Science  Monthly,  Vol.  XI,  p.  316 ; 
"Appleton's  Cyclopaedia,"  Articles  on  Hydromechanics,  Atmosphere, 
Pneumatics,  etc.  Delaunay,  "  Mecanique  Rationnelle";  Boutan  et 
D' Almeida,  "Cours  de  Physique";  Miiller,  "  Lehrbuch  der  Physik 
und  Meteorologie";  Miiller,  "Lehrbuch  der  Kosmischen  Physik"; 
Wiillner,  "Lehrbuch  der  Experimental  Physik ";  Mousson,  "Die 
Physik  auf  Grundlage  der  Erfahrung " ;  Beetz,  "  Leitfaden  der 
Physik";  Kuelp,  "  Die  Schule  des  Physikers." 

On  the  theory  of  Wave-Motion,  and  the  subjects  of  Sound  and 
Light,  which  are  now  to  follow,  consult  Lockyer's  "  Studies  in  Spec- 
trum Analysis";  Lloyd's  "Wave  Theory";  Taylor's  "Sound  and 
Harmony";  Blaserna's  "Theory  of  Sound  in  Relation  to  Music"; 
Tyndall's  "Sound"  and  "Light";  Lockyer's  "Water-waves  and 
Sound-waves  "  in  Popular  Science  Monthly,  Vol.  XIII,  p.  1 66 ;  Shaw's 
"  How  Sound  and  Words  are  Produced,"  in  Popular  Science  Monthly, 
Vol.  XIII,  p.  43;  Schellen's  "Spectrum  Analysis";  Airy's  Optics; 
Lockyer's  Spectroscope  ;  Chevreul's  Colors  ;  Spottiswoode's  "  Polariza- 
tion of  Light";  Lommers  "  Nature  of  Light";  Helmholtz's  "  Popular 
Lectures  on  Scientific  Subjects";  "Appleton's  Cyclopaedia,"  Articles 
on  Sound,  Light,  Spectrum,  Spectrum  Analysis,  Spectacles,  Heat,  etc. ; 
Stokes's  "Absorption  and  Colors,"  and  Forbes's  "Radiation,"  in 
Science  Lectures  at  South  Kensington,  Vol.  I ;  Mayer  and  Barnard's 
Light;  Draper's  "Popular  Exposition  of  some  Scientific  Experi- 
ments," in  Harper's  Magazine  for  1877  ;  Core's  "  Modern  Discoveries 
in  Sound,"  in  Manchester  Science  Lectures,  '77-8  ;  Dolbeare's  "  Art  of 
Projecting";  Draper's  "Scientific  Memoirs";  Steele's  Physiology, 
Section  on  Sight,  pp,  187-196. 


VI. 
Oj*.  So 


"  Scitnce  ought  to  teach  us  to  see  the  invisible  as  well  as  the  visible  in 
nature  :  to  picture  to  our  mind's  eye  those  operations  that  entirely  elude  the 
eye  of  the  body  ;  to  look  at  the  very  atoms  of  matter,  in  motion  and  in  rest, 
and  to  follow  them  forth  into  the  world  of  the  senses!' — TYNDALL. 


ANALYSIS. 


r   1  PRODUCTION  OF  SOUND. 


o 

OQ 


TRANSMISSION 
SOUND. 


OF 


(1.)  Through  air. 
(2.)  In  a  vacuum. 


(3.)  Velocity. 


(4.)  Intensity. 


8.  REFRACTION  OF  SOUND. 

J(l.)  Law  of. 
(2.)  Echoes. 
SOUND. 


(a.)  Depends     on 

what? 

(b.)  Rate  in  air. 
(c.)  Bate  in  water. 
(d.)  Rate  in  solids. 
(e.)  Velocity  is 

uniform. 
(f.)  Used  to  find 

distance. 
(a.)  Depends 

what? 
(b.)  Law  of. 
(c.)  Speaking 

tubes,  etc. 


on 


5.  MUSICAL  SOUNDS. 


(3.)  Decrease  of  Intensity. 

(4.)  Acoustic  Clouds. 

(1.)  Difference  between    Noise 

Music. 
(2.)  Pitch. 

(3.)  To  find  Number  of  Waves. 
(4.)  To  find  Length  of  Waves. 
(5.)  Unison. 
6.  SUPER-POSITION  OF  SOUND-WAVES. 
—  Definitions. 
(1.)  Sonometer. 
(2.)  Three  Laws. 
(3.)  Nodes. 

(4.)  Acoustic  Figures. 
(5.)  Harmonics. 
(6.)  Nodes  of  Bell. 
(7.)  Nodes  of  Sounding-Board. 
(8.)  Musical  Scale. 


and 


7.  VIBRATION 
CORDS. 


OF 


8.  WIND  INSTRUMENTS. 


SYMPATHETIC 
BRATIONS. 


VI- 


10.  THE  PHONOGRAPH. 

11.  THE  EAR. 


—  Illustrations. 

(1.)  Sensitive  Flames. 
(2.)  Singing  Flames. 

—  Description. 
(1.)  Range  of. 

(2.)  Ability  to  Detect  Sound. 


ACOUSTICS,    OR    THE     SCIENCE 
OF    SOUND.* 

1.  Production  of  Sound. — By  lightly  tapping  a  glass 
fruit-dish,  we  can  throw  the  sides  into  motion  visible  to  the 
eye. — Fill  a  goblet  half -full  of  water,  and  rub  a  wet  finger 
lightly  around  the  upper  edge  of  the  glass.  The  sides  will 
vibrate,  and  cause  tiny  waves  to  ripple  the  surface  of  the 
water. — Hold  a  card  close  to  the  prongs  of  a  vibrating 
tuning-fork,  and  you  can  hear  the  repeated  taps.  Place  the 
cheek  near  them,  and  you  will  feel  the 
little  puffs  of  wind.  Insert  the  handle 
between  your  teeth, 
and  you  will  expe- 
rience the  indescrib- 
able 
thrill  J 
of  the 

swinging  metal.  The  tuning-fork  may  be  made  to  draw 
the  outline  of  its  vibrations  upon  a  smoked-glass.  Fasten 
upon  one  prong  a  sharp  point,  and  drawing  the  fork  along, 
a  sinuous  line  will  show  the  width  (amplitude)  of  the 
vibrations. 

*  The  term  sound  is  used  in  two  senses— the  subjective  (which  has  reference  to  our 
mind)  and  the  objective  (which  refers  to  the  objects  around  us).  (1.)  Sound  is  the 
sensation  produced  upon  the  organ  of  hearing  by  vibrations  in  matter.  In  this  use 
of  the  word  there  can  be  no  sound  where  there  is  no  ear  to  catch  the  vibrations.— 
An  oak  falls  in  the  forest,  and  if  there  is  no  ear  to  hear  it  there  is  no  noise,  and  the 
old  tree  drops  quietly  to  its  resting-place. — Niagara's  flood  poured  over  its  rocky 
precipice  for  ages,  but  fell  silently  to  the  ground.  There  were  the  vibrations  of  earth 
and  air,  but  there  was  no  ear  to  receive  them  and  translate  them  into  sound.  When 
the  first  foot  trod  the  primeval  solitude,  and  the  ear  felt  the  pulsations  from  the 
torrent,  then  the  roaring  cataract  found  a  voice  and  broke  its  lasting  silence.— A 
trumpet  does  not  sound.  It  only  carves  the  air  into  waves.  The  tympanum  is  the 
beach  on  which  these  break  into  sound.  (2.)  Sound  is  those  vibrations  of  matter 
capable  of  producing  a  sensation  upon  the  organ  of  hearing.  In  this  use  of  the  word 
there  can  be  a  sound  in  the  absence  of  the  ear.  An  object  falls  and  the  vibrations  are 
produced,  though  there  may  be  no  organ  of  hearing  to  receive  an  impression  from 
them.  This  is  the  sense  in  which  the  term  sound  is  commonly  used. 


124 


ACOUSTICS. 


2.  Transmission  of  Sound. — (1.)  THROUGH  AIR. 
The  prong  of  a  tuning-fork  advances  condensing  the  air  in 
front,  and  then  recedes,  leaving  behind  it  a  partial  vacuum. 
This  process  is  repeated  until  the  fork  comes  to  rest,  and 
the  sound  ceases.  Each  vibration  produces  a  sound-wave  of 
air,  which  contains  one  condensation  and  one  rarefaction. 
In  water,  we  measure  a  wave-length  from  crest  to  crest ;  in 
air,  from  condensation  to  condensation.  The  condensation 
of  the  sound-wave  corresponds  to  the  crest,  and  the  rarefac- 
tion of  the  sound-wave  to  the  hollow  of  the  water-wave.  In 


JL    a 


FIG.  117. 

6'  c 


Fig.  117,  the  dark  spaces  a,  b,  c,  d  represent  the 
condensations,  and  a',  b't  c'  the  rarefactions ;  the 
wave-lengths  are  the  distances  ab,  be,  cd. 

If  we  fire  a  gun,  the  gases  which  are  produced 
expand  suddenly  and  force  the  air  outward  in 
every  direction.     This  hollow  shell  of  condensed  air  imparts 
its  motion  to  that  next,  while  it  springs  back  by  its  elasticity 

FIG.  118. 


and  becomes  rarefied.    The  second  shell  rushes  forward  with 


TRANSMISSION    OF    SOUND. 


125 


FIG.  119. 


the  motion  received,  then  bounds  back  and  becomes  rarefied. 
Thus  each  shell  of  air  takes  up  the  motion  and  imparts  it  to 
the  next.  The  wave,  consisting  of  a  condensation  and  a 
rarefaction,  proceeds  onward.  It  is,  however,  as  in  water- 
waves,  a  movement  of  the  form  only,  while  the  particles 
vibrate  but  a  short  distance  to  and  fro.  The  molecules  in 
water-waves  oscillate  vertically ;  those  in  sound-waves  hori- 
zontally, or  parallel  to  the  line  of  motion. 

If  a  bell  le  rung,  the  adjacent 
air  is  set  in  motion  ;  thence,  by  a 
series  of  condensations  and  rare- 
factions, the  vibrations  are  con- 
veyed to  the  ear. 

When  we  speak,  we  do  not  shoot 
the  air  we  expel  from  our  lungs 
into  the  ear  of  the  listener.  "We 
simply  condense  the  air  before  the 
mouth  and  throw  it  into  vibra- 
tion. Thus  a  sound-wave  is 
formed.*  This  spreads  in  every 
direction  in  the  form  of  a  sphere 
of  which  we  are  the  centre,  f 

(2.)  IN  A  VACUUM.  The  bell 
B  (Fig.  119)  may  be  set  in  mo- 
tion by  the  sliding-rod  r.  The 
apparatus  is  suspended  by  silk 
cords,  that  no  vibration  may  be 
conducted  through  the  pump.  If 
the  air  be  exhausted,  the  sound 

*  A  continuous  blast  of  air  produces  no  sound.  The  rush  of  the  grand  ae"rial 
rivers  above  us  we  never  hear.  They  flow  on  in  the  upper  regions  ceaselessly  hut 
silently.  A  whirlwind  is  noiseless.  Let,  however,  the  great  billows  strike  a  tree  and 
wrench  it  from  the  ground,  and  we  can  hear  the  secondary,  shorter  waves  which  set 
out  from  the  struggling  limbs  and  the  tossing  leaves. 

t  "  It  is  marvellous,"  says  Youmans,  "  how  slight  an  impulse  throws  a  vast 
amount  of  air  into  motion.  We  can  easily  hear  the  song  of  a  bird  500  feet  above  us. 
For  its  melody  to  reach  us  it  must  have  filled  with  wave-pulsations  a  sphere  of  air 
1,000  feet  in  diameter,  or  set  in  motion  18  tons  of  the  atmosphere.11 


12G  ACOUSTICS. 

will  become  so  faint  that  it  cannot  be  heard,  even  when  the 
ear  is  placed  close  to  the  receiver.* 

In  elevated  regions  sounds  are  diminished  in  loudness, 
and  it  is  difficult  to  carry  on  a  conversation,  as  the  voice 
must  be  raised  so  high.  The  reverse  takes  place  in  deep 
mines  and  diving-bells.  The  sounds  then  become  start- 
lingly  distinct,  and  workmen  are  enabled  to  converse  audibly 
in  whispers. 

(3.)  THE  VELOCITY  OF  SOUND  depends  on  the  ratio  of  the 
elasticity  to  the  density  of  the  medium  through  which  it 
passes.  The  higher  the  elasticity,  the  more  promptly  and 
rapidly  the  motion  is  transmitted,  since  the  elastic  force 
acts  like  a  bent  spring  between  the  molecules;  and  the 
greater  the  density,  the  more  molecules  to  be  set  in  motion, 
and  hence  the  slower  the  transmission. 

Sound  travels  through  air  (at  32°  F.)  1,090  feet  per  second. 
A  rise  in  temperature  diminishes  the  density  of  the  air, 
and  thus  increases  the  velocity  of  sound.  A  difference  of 
1°  F.  makes  a  variation  of  about  1  foot.  Sound  also  moves 
faster  in  damp  than  in  dry  air. 

Sound  travels  through  water  about  4^00  feet  per  second. 
Water  being  denser  than  air  should  convey  sound  more 
slowly  ;  but  its  high  elasticity  (p.  20)  quadruples  the  rate. 

Sound  travels  through  solids  faster  than  through  air. 
This  may  be  illustrated  by  placing  the  ear  close  to  the  hori- 
zontal bar  at  one  end  of  an  iron  fence,  while  a  person 
strikes  the  other  end  a  sharp  blow.  Two  sounds  will  reach 
the  ear — one  through  the  metal,  and  afterward  another 
through  the  air.  The  velocity  varies  with  the  nature  of  the 
solid,  f  In  the  metals  it  is  from  4  to  16  times  that  in  air. 

*  There  would  be  perfect  silence  in  a  perfect  vacuum.  No  pound  is  transmitted 
to  the  earth  from  the  regions  of  space.  The  movements  of  the  heavenly  bodies  are 
noiseless.  In  the  expressive  language  of  David,  "  Their  voice  is  not  heard." 

t  Wheats  tone  invented  a  beautiful  experiment  to  show  the  transmission  of  sound 
through  wood.  Upon  the  top  of  a  music-box,  he  rested  the  end  of  a  wooden  rod 
reaching  to  the  room  above,  and  insulated  from  the  ceiling  by  India  rubber.  A  violin 
^  placed  on  the  top  of  the  rod.  the  sounds  from  the  box  below  filled  the  upper 


TRANSMISSION    OF    SOUND.  127 

Different  sounds  travel  with  the  same  velocity.*  A  band 
may  be  playing  at  a  distance,  yet  the  harmony  of  the  dif- 
ferent instruments  is  preserved.  The  soft  and  the  loud,  the 
high  and  the  low  notes  reach  the  ear  at  the  same  time. 

Velocity  of  sound  used  to  find  distance.  Light  travels 
instantaneously  so  far  as  all  distances  on  the  earth  are 
concerned.  Sound  moves  more  slowly.  We  see  a  chopper 
strike  with  his  axe,  and  a  moment  elapses  before  we  hear 
the  blow.  If  one  second  intervenes  the  distance  is  about 
1,090  feet.  By  means  of  the  second  hand  of  a  watch  or  the 
beating  of  our  pulse,  we  can  count  the  seconds  that  elapse 
between  a  flash  of  lightning  and  the  peal  of  thunder  which 
follows.  Multiplying  the  velocity  of  sound  by  the  number 
of  seconds,  we  obtain  the  distance  of  the  thunderbolt. 

(4.)  THE  INTENSITY  OF  SOUND  is  proportional  to  the 
square  of  the  amplitude,  i.  e.9  the  arc  through  which  the 
molecules  swing  to  and  fro.  As  in  a  pendulum,  the  greater 
the  amplitude  the  greater  the  velocity.  The  force  of  a 
striking  body  depends  upon  its  mass  and  the  square  of  its 
velocity  (p.  36).  So  one  sound  is  louder  than  another,  be- 
cause the  air  molecules  hit  the  ear-drum  with  greater  force. 
On  the  top  of  a  mountain,  because  of  the  rare  atmosphere, 
there  are  fewer  molecules  to  strike  the  ear ;  hence,  the  blow 
is  less  intense. 

Tlie  intensity  of  sound  diminishes  as  the  square  of  the 
distance  increases.}  The  sound-wave  expands  in  the  form 

room,  appearing  to  emanate  from  the  violin. — Take  two  small,  round,  tin  boxes  and 
pass  a  strong  string  of  any  length  through  a  hole  in  the  bottom  of  each,  fastening  it 
by  a  knot.  If  the  string  be  drawn  tightly,  and  one  box  be  held  to  the  mouth  of  the 
speaker  and  the  other  to  the  ear  of  the  listener,  the  faintest  whisper  can  be  heard. 

*  It  has  been  said  that  the  "  heaviest  thunder  travels  no  faster  than  the  softest 
whisper."  Mallet,  however,  found  that  in  blasting  with  a  charge  of  2,000  Ibs.,  the 
velocity  was  967  feet  per  second,  while  with  12,000  Ibs.  it  was  Increased  to  1,210  feet. 
Parry  in  his  Arctic  travels  states  that,  on  a  certain  occasion,  the  sound  of  the  sunset- 
gun  reached  his  ears  before  the  officer's  word  of  command  to  fire,  proving  that  the 
report  of  the  cannon  travelled  sensibly  faster  than  the  sound  of  the  voice. 

t  The  same  proportion  obtains  in  Gravitation,  Sound,  Light,  and  Heat.  We  have 
seen  how  the  Pendulum  Is  based  upon  the  force  of  Gravity,  and  reveals  the  Laws  of 
Falling  Bodies.  Now  we  find  that  the  Pendulum,  and  even  the  principles  of  Reflected 


128  ACOUSTICS. 

of  a  sphere.  The  larger  the  sphere,  the  greater  the  number 
of  air  particles  to  be  set  in  motion,  and  the  feebler  their 
vibration.  The  surfaces  of  spheres  are  proportional  to  the 
squares  of  their  radii ;  the  radii  of  sound-spheres  are  their 
distances  from  the  centre  of  disturbance.  Hence  the  force 
with  which  the  molecules  will  strike  the  ear  decreases  as  the 
square  of  our  distance  from  the  sounding  body. 

Speaking-tubes  conduct  sound  to  distant  rooms  because 
they  prevent  the  waves  from  expanding  and  losing  their 
intensity.*  The  ear-trumpet  collects  waves  of  sound  and 
reflects  them  into  the  ear.  The  speaking-trumpet  is  based 
on  the  same  principle  as  the  speaking-tube.  Probably  also 
the  sound  of  the  voice  is  strengthened  by  the  vibrations  of 
the  air  in  the  tube. 

3.  Refraction  of  Sound.— When  a  sound-wave  goes 
obliquely  from  one  medium  to  another,  it  is  bent  out  of  its 
course.  Like  light,  it  may  be  passed  through  a  lens  and 

FIG.  120. 


brought  to  a  focus.  B  is  a  rubber  globe,  filled  with  carbonic- 
acid  gas ;  w  is  a  watch,  and  /'  a  funnel  which  assists  in 
collecting  the  wave  at/,  where  the  ear  is  placed.  The  ticks 

Motion  and  Momentum,  are  linked  with  the  phenomena  of  Sound.  As  we  progress 
farther,  we  shall  find  how  Nature  is  thus  interwoven  everywhere  with  proofs  of  a 
common  plan  and  a  common  Author. 

*  Biot  held  a  conversation  through  a  Paris  water-pipe  8,120  feet  long.  He  says 
that  "  it  was  so  easy  to  be  heard,  that  the  only  way  not  to  be  heard  was  not  to  speak 
atalL" 


REFLECTION    OP    SOUKD. 

of  the  watch  can  be  heard,  while  outside  the  focus  they  are 
inaudible. 

4.  Reflection  of  Sound. — When  a  sound-wave  strikes 
against  the  surface  of  another  medium,  a  portion  goes  on 
while  the  rest  is  reflected. 

(1.)  THE  LAW  is  that  of  Motion ; — the  angle  of  incidence 
is  equal  to  that  of  reflection.*  If  the  reflecting  surface  be 
very  near,  the  reflected  sound  will  join  the  direct  one  and 
strengthen  it.  This  accounts  for  the  well-known  fact  that 
a  speaker  can  be  heard  more  easily  in  a  room  than  in  the 
open  air,  and  that  a  smooth  wall  back  of  the  stand  re- 
inforces the  voice.  The  old-fashioned  "sounding-boards" 
were  by  no  means  inefficient,  however  singular  may  have 
been  their  appearance.  Shells,  by  their  peculiar  convolu- 
tions, reflect  the  various  sounds  which  fill  even  the  stillest 
air.  As  we  hold  them  to  our  ear,  they  are  poetically  said  to 
"repeat  the  murmurs  of  their  ocean  home." 

(2.)  ECHOES  are  produced  where  the  reflecting  surface 
is  so  distant  that  we  can  distinguish  the  reflected  from  the 
direct  sound.  If  the  sound  be  short  and  quick,  this  requires 
at  least  56  feet ;  but  if  it  be  an  articulate  one,  112  feet  are 
necessary.  One  can  pronounce  or  hear  distinctly  about  five 
syllables  in  a  second ;  1,120  ft.  (the  velocity  at  a  medium 
temperature)  -j-  5  =  224  ft.f  If  the  wave  travel  224  feet 

*  Domes  and  curved  walls  reflect  sound  as  mirrors  do  light.  Thus,  in  the  gallery 
under  the  dome  of  St.  Paul's  Cathedral,  London,  persons  standing  close  to  the  wall 
can  whisper  to  each  other  and  be  heard  at  a  great  distance.— Two  persons,  placed 
with  their  backs  to  each  other,  at  the  foci  of  an  oval  room,  or  "  Whispering  Gallery," 
can  carry  on  a  conversation  that  will  be  inaudible  to  spectators  standing  between 
them.— The  covered  recesses  on  the  opposite  sides  of  a  street,  or  the  arches  of  a 
stone  bridge,  oftentimes  reflect  sound  so  as  to  enable  persons  seated  at  the  foci  to 
converse  in  whispers  while  loud  noises  are  being  made  in  the  open  space  between 
these  semi-domes. 

t  When  several  parallel  surfaces  are  properly  situated,  the  echo  may  be  repeated 
backward  and  forward  in  a  surprising  manner.  In  Princeton,  Ind.,  there  is  an  echo 
between  two  buildings  that  will  return  the  word  "  Knickerbocker "  twenty  times. 
So  many  persons  yisited  the  place  that  the  city  council  forbade  the  use  of  the  echo 
after  9  o'clock  at  night.— At  Woodstock,  England,  an  echo  returns  17  syllables  by  day 
and  30  by  night.  The  reflecting  surface  is  distant  about  2.300  feet,  and  a  sharp  ha! 


130  ACOUSTICS. 

in  going  and  returning,  the  two  sounds  will  not  blend,  and 
the  ear  can  detect  an  interval  between  them.  A  person 
speaking  in  a  loud  roice  in  front  of  a  mirror  112  feet  dis- 
tant, can  distinguish  the  echo  of  the  last  syllable  he  utters  ; 
at  224:  feet,  the  last  two  syllables,  etc. 

(3.)  DECEEASE  or  SOUND  BY  KEFLECTION. — If  we  strike 
the  bell  represented  in  Fig.  119,  we  shall  find  a  marked  dif- 
ference between  its  sound  under  the  glass  receiver  and  in 
the  open  air.  Floors  are  deadened  with  tan-bark  or  mortar, 
since  as  the  sound-wave  passes  from  particle  to  particle  of 
the  unhomogeneous  mass,  it  becomes  weakened  by  partial 
reflection.  The  air  at  night  is  more  homogeneous,  and 
hence  sounds  are  heard  further  and  more  clearly  than  in  the 
day  time. 

(4. )  ACOUSTIC  CLOUDS  are  masses  of  moist  air  of  varying 
density,  which  act  upon  sounds  as  common  clouds  do  upon 
light,  wasting  it  by  repeated  reflections.  They  may  exist  in 
the  clearest  weather.  To  their  presence  is  to  be  attributed 
the  variation  often  noticed  in  the  distance  at  which  well- 
known  sounds,  as  the  ringing  of  church  bells,  blowing  of 
engine-whistles,  etc.,  are  heard  at  different  times.* 

will  come  back  a  ringing  ha,  ha,  ha  /—The  echo  is  often  softened,  as  in  the  Alpine 
regions,  where  it  warbles  a  beautiful  accompaniment  to  the  shepherd's  horn.— The 
celebrated  echo  of  the  Metelli  at  Rome  was  capable  of  distinctly  repeating  the  first 
line  of  the  ^Eneid  8  times.— In  Fairfax  County,  Va.,  is  an  echo  which  will  retuni  20 
notes  played  on  a  flute,  but  supplies  the  place  of  some  notes  with  their  thirds,  fifths, 
or  octaves.— The  tick  of  a  watch  may  be  heard  from  one  end  of  the  Church  of  St. 
Albans  to  the  other.— At  Carisbrook  Castle,  fele  of  Wight,  is  a  well  210  feet  deep  and 
12  feet  wide,  lined  with  smooth  masonry.  When  a  pin  is  dropped  into  the  well  it  is 
distinctly  heard  to  strike  the  water.— In  certain  parts  of  the  Colosseum  at  London 
the  tearing  of  paper  sounds  like  the  patter  of  hail,  while  a  single  exclamation  comes 
back  a  peal  of  laughter.— The  Dome  of  the  Baptistry  of  the  Cathedral  at  Pisa  (See 
Frontispiece)  has  a  wonderful  echo.  During  some  experiments  there,  the  author 
found  every  noise,  even  the  rattle  of  benches  on  the  pavement  below,  to  be  reflected 
back  as  if  from  an  immense  distance  and  to  return  mellowed  and  softened  into 
music  (note,  p.  131).  See  also  p.  xi.,  Fresh  Facts  and  Theories. 

*  The  extinction  of  sound  by  such  agencies  Is  iften  almost  incredible.  Thus  two 
observers  looking  across  the  valley  of  the  Chickahominy  at  the  battle  of  Gaines's 
Mill  failed  to  hear  a  sound  of  the  conflict,  though  they  could  clearly  see  the  lines  of 
soldiers,  the  batteries  and  the  flash  of  the  guns.— These  phenomena  are  ascribed 
by  many  (page  264)  to  an  elevation  or  a  depression  of  the  wave-front  so  that  the 
Bound  passes  above  the  observer  or  is  stopped  before  it  reaches  him.  See  Stewart's 
Physics,  p.  141. 


REFLECTION    OF    SOUND. 


131 


5.  Musical  Sounds. — (1.)  THE  DIFFERENCE  BETWEEN 
NOISE  AND  MUSIC  is  that  between  irregular  and  regular 
vibrations.  Whatever  the  cause  which  sets  the  air  in  mo- 
tion, if  the  vibrations  are  uniform  and  rapid  enough,  the 
sound  is  musical.  If  the  ticks  of  a  watch  could  be  made 
with  sufficient  rapidity,  they  would  lose  their  individuality 
and  blend  into  a  musical  tone.  If 
the  puffs  of  a  locomotive  could 
reach  50  or  60  a  second,  its  ap- 
proach would  be  heralded  by  a  tre- 
mendous organ-peal.* 

(2.)  PITCH  depends  on  the  rapid- 
ity of  the  vibrations.  Thus  if  we 
hold  a  card  against  the  cogs  of  the 
rapidly-revolving  wheel  in  the  appa- 
ratus shown  in  Fig.  16,  we  shall  ob- 
tain a  clear  tone  ;  and  the  faster  the 
wheel  turns,  the  shriller  the  tone, 
i.  e.y  the  higher  the  pitch. 

(3.)   THE  NUMBER  OF  WAVES  IN  A 

SOUND  is  determined  by  an  instru- 
ment called  the  Siren.  0  is  a  cylin- 
drical box ;  #,  a  pipe  for  admitting 
air ;  db,  a  plate  pierced  with  four 
series  of  holes,  containing  8,  10,  12, 
and  16  orifices  respectively  ;  m,  n, 
o,p  are  stops  for  closing  any  series. 

*  The  pavement  of  London  is  largely  composed  of  granite  blocks,  four  inches  in 
width.  A  cab-wheel  jolting  over  this  at  the  rate  of  eight  miles  per  hour  produces  a 
succession  of  35  sounds  per  second.  These  link  themselves  into  a  soft,  deep  musical 
tone,  that  will  bear  comparison  with  notes  derived  from  more  sentimental  sources. 
This  tendency  of  Nature  to  music  is  something  wonderful.  "Even  friction,"  says 
Tyndall,  "  is  rhythmic."  A  bullet  flying  through  the  air  sings  softly  as  a  bird.  The 
limbs  and  leaves  of  trees  murmur  as  they  sway  in  the  breeze.  The  rumble  of  a  great 
city,  all  the  confused  noises  of  Nature  when  softened  by  distance,  are  said  to  be  on 
one  pitch—the  key  of  F.  Falling  water,  singing  birds,  sighing  winds,  everywhere 
attest  that  the  same  Divine  love  of  the  beautiful  which  causes  the  rivers  to  wind 
through  the  landscape,  the  trees  to  bend  in  a  graceful  curve— the  line  of  beauty— and 
the  rarest  flowers  to  bud  and  blossom  where  no  eye  save  His  may  see  them,  delights 
also  in  the  anthem  of  praise  which  Nature  sings  for  His  ear  alone. 


132 


ACOUSTICS. 


FIG.  122. 


The  rod  p  is  bevelled  at  p1  so  as  to  turn  in  the  socket  x ; 
de  is  a  disk  with  holes  corresponding  to  those  in  the  lower 
plate,  over  which  it  revolves.  At  s  is  an  endless  screw, 
which  causes  two  wheels  to  rotate,  and  thus  turns  the  hands 

upon  the  dial  (Fig.  122).  On  this  we 
can  see  the  number  of  revolutions 
made  by  the  upper  disk.  The  holes 
in  ab  and  de  are  inclined  to  each 
other,  so  that,  when  a  current  of  air 
is  forced  in  at  t,ii  passes  up  through 
the  openings  in  the  lower  disk,  and 
striking  against  the  sides  of  those  in 
the  upper  disk,  causes  it  to  revolve. 
As  that  turns,  it  alternately  opens 
and  closes  the  orifices  in  the  lower 
disk,  and  thus  converts  the  steady 
stream  of  air  into  uniform  puffs. 
At  first  they  succeed  each  other  so 
slowly  that  they  may  easily  be 
counted.  But,  as  the  motion  in- 
creases, they  link  themselves  to- 
gether, and  burst  into  a  full,  melo- 
dious note.  As  the  velocity  aug- 
ments, the  pitch  rises,  until  the  mu- 
sic becomes  painfully  shrill.  Di- 
minish the  speed,  and  the  pitch  falls. 

To  find,  therefore,  the  number  of  vibrations  in  a  given 
sound,  force  the  air  through  the  Siren  until  the  required 
pitch  is  reached.  See  on  the  dial,  at  the  end  of  a  minute, 
the  number  of  revolutions  of  the  disk.  When  the  row  con- 
taining ten  holes  is  open,  and  the  tone  C2,  it  will  indicate 
1,536.  There  were  ten  puffs  of  air,  or  ten  waves  of  sound, 
in  each  revolution.  1,536  x  10  =  15,360.  Dividing  this 
by  60,  we  have  256,  the  number  per  second.  When  the 
inner  and  outer  rows  of  holes  are  opened,  the  ear  detects  the 
difference  of  an  octave  between  the  two  sounds.  The  one 
containing  8  produces  the  lower,  and  16  the  higher  tone. 


MUSICAL    BOUNDS.  133 

Hence  an  octave  of  a  tone  is  caused  by  double  the  number  of 
vibrations. 

(4.)  To  FIND  THE  LENGTH  OF  THE  WAVE. — Suppose  the 
air  in  the  last  experiment  was  of  such  a  temperature  that 
the  foremost  sound-wave  travelled  1,120  feet  in  a  second. 
In  that  space  there  were  256  sound-waves.  Dividing  1,120 
by  256,  we  have  4J-  feet  as  the  length  of  each.  We  thus  find 
the  wave-length  by  dividing  the  velocity  by  the  number  of 
vibrations  per  second.  As  the  pitch  is  elevated  by  rapidity 
of  vibration,  we  perceive  that  the  low  tones  in  music  are 
produced  by  the  long  waves  and  the  high  tones  by  the  short 
ones.* 

(5.)  TONES  IN  UNISON.— If  the  string  of  a  violin,  the 
cord  of  a  guitar,  the  parchment  of  a  drum,  and  the  pipe  of 
an  organ,  produce  the  same  tone,  it  is  because  they  are  exe- 
cuting the  same  number  of  vibrations  per  second.  If  a 
voice  and  a  piano  perform  the  same  music,  the  steel  strings 
of  the  piano  and  the  vocal  cords  of  the  singer  vibrate  to- 
gether and  send  out  sound-waves  of  the  same  length,  f 

6.  Super-position  of  Sound-waves. — The  air  may 
transmit  sound-waves  from  a  thousand  instruments  at  once. 
If  the  condensation  of  one  wave  meet  the  condensation  of 
another,  the  sound  will  be  augmented,  the  condensations 
becoming  more  condensed  and  the  rarefactions  more  rarefied 
(p.  102).  If  the  condensation  of  one  meet  the  rarefaction 

*  The  aerial  waves  are  seemingly  shortened  when  the  source  of  sound  is  approach- 
Ing,  whether  by  its  own  motion  or  the  hearer's,  and  lengthened  when  it  is  receding. 
In  the  former  case  the  tone  of  the  sound  is  more  acute,  in  the  latter  graver.  This  is 
strikingly  illustrated  when  a  swift  train  rushes  past  a  station,  the  whistle  blowing. 
While  the  cars  are  approaching,  a  person  hears  a  note  somewhat  sharper ;  after  it 
has  passed,  one  somewhat  flatter  than  the  true  note.  Still  more  obvious  is  the 
change  when  two  trains  pass  each  other.  A  person  unfamiliar  with  the  arrangement 
would  suppose  a  different  bell  was  rung.  In  one  case  more  and  in  the  other  fewer 
waves  reach  the  ears  in  a  second.  Just  so  a  ship  moving  against  the  sea  meets  more 
waves  than  one  moving  with  it. 

t  In  order  to  determine  the  number  and  length  of  the  sound-waves  produced  by  a 
eonorous  body,  we  have  only  to  bring  its  sound  and  that  of  the  siren  hi  unison. 
"  The  wings  of  a  gnat  flap,  in  flying,  at  the  rate  of  15,000  times  per  second.  A  tired 
bee  hums  on  E,  while  in  pursuit  of  honey  it  hums  contentedly  on  A.  The  common 
horse-fly  moves  its  wings  335  times  a  second ;  a  honey-bee,  190  times." 


134 


ACOUSTICS. 


of  the  other,  one  wave-motion  will  be  striving  to  push  the 
air  molecules  forward,  and  the  other  to  urge  them  back- 
ward. So  that,  if  they  meet  in  exactly  opposite  phases  and 
the  two  forces  are  equal,  they  will  balance  each  other  and 
silence  will  ensue.* 

FIG.  123. 


1HB 


Suppose  we  have  two  tuning-forks,  A  and  B,  a  wave- 
length apart,  and  vibrating  in  unison.  The  waves  will 
coincide,  as  represented  by  the  light  and  dark  shades  in 
Fig.  123.  The .  same  result  would  occur  if  they  were  any 
number  of  wave-lengths  apart.  If  they  are  a  half  wave- 
length apart,  the  condensation  of  A  will  coincide  with  the 
rarefaction  of  B,  and  vice  versa.  The  effect  is  represented 

FIG.  124. 


by  the  uniformity  of  the  shading  in  Fig.   124.     This  is 
termed  interference  of  sound-waves.] 

7.  Vibrations  of  Cords.— Let  ab  be  a  stretched  cord 
made  to  vibrate.  The  motion  from  e  to  d  and  back  again 
is  termed  a  vibration;  that  from  e  to  d,  a  half-vibration. 

*  Thus  a  sound  added  to  a  sound  may  produce  silence.  In  the  same  way,  two 
motions  may  produce  rest ;  two  lights  may  cause  darkness ;  and  two  heats  may 
produce  cold. 

t  If  we  strike  a  tuning-fork  and  turn  it,  slowly  around  before  the  ear,  we  shall  find 
four  points  where  the  interference  of  the  sound-waves  neutralizes  the  vibrations  and 
causes  silence.— Two  forks  or  organ-pipes  nearly  in  unison,  produce  the  well-known 
-  beats,"  a  characteristic  phenomenon  of  interference. 


VIBRATIONS    OF    CORDS. 


135 


The  intensity  of  the  sound  depends  on  the  width  of  ed,  i.  e., 
the  amplitude  of  the  vibration. 

(1.)  THE  SONOMETER  is  an  instrument  used  to  investigate 
the  laws  which  govern  the  vibrations.  It  consists  of  two 
cords  stretched  by  weights,  P,  across  fixed  bridges,  AB. 


FIG.  136, 


The  movable  bridge,  D,  serves  to  lengthen  or  shorten  the 
cords.  Beneath  is  a  wooden  box  which  communicates  the 
vibrations  of  the  cords  to  the  air  within.  This  is  the  real 
sounding  body. 

(2.)  THREE  LAWS. — I.  The  number  of  vibrations  per  sec- 
ond increases  as  the  length  of  the  cord  decreases.  With  the 
bow  make  the  cord  vibrate,  giving  the  note  of  the  entire 
string.  Place  the  bridge  D  at  the  centre  of  the  cord,  and 
the  sound  will  be  the  octave  above  the  former.  Thus  by 
taking  one-half  the  length  of  the  cord  we  double  the  num- 
ber of  vibrations.  Ex. :  If  an  entire  cord  make  20  vibrations 
per  second,  one-half  will  make  40,  and  one-third,  60. — The 
violin  or  guitar  player  elevates  the  pitch  of  a  string  by  mov- 
ing his  finger,  thus  shortening  the  vibrating  portion. — In 


136  ACOUSTICS. 

the  piano,  harp,  etc.,  the  long  and  the  short  strings  produce 
the  low  and  the  high  notes  respectively. 

II.  The  number  of  vibrations  per  second  increases  as  the 
square  root  of  the  tension.    The  cord  when  stretched  by 
1  Ib.  gives  a  certain  tone.     To  double  the  number  of  vibra- 
tions and  obtain  the  octave  requires  4  Ibs.     Stringed  instru- 
ments are  provided  with  keys,  by  which  the  tension  of  the 
cord  and  the  corresponding   pitch  may  be  increased  or 
diminished. 

III.  The  number  of  vibrations  per  second  decreases  as  the 
square  root  of  the  weight  of  the  cord  increases.     If  two  strings 
of  the  same  material  be  equally  stretched,  and  one  have  four 
times  the  weight  of  the  other,  it  will  vibrate  only  half  as 
often.    In  the  violin  the  bass  notes  are  produced  by  the 
thick  strings.     In  the  piano  fine  wire  is  coiled  around  the 
heavy  strings. 

(3.)  NODES. — In  these  experiments,  the  cord  is  shortened 
by  a  movable  bridge  which  holds  it  firmly.     If,  instead,  we 

Fie.  127. 


rest  a  feather  lightly  on  the  string,  and  draw  the  bow  over 
one-half,  the  cord  will  vibrate  in  two  portions  and  give  the 
octave  as  before.  Remove  the  feather,  and  it  will  continue 
to  vibrate  in  two  parts  and  to  yield  the  same  tone.  We  can 
show  that  the  second  half  vibrates  by  placing  across  that 
portion  a  little  paper  rider,  On  drawing  the  bow  it  will  be 


VIBRATIONS    OF    CORDS. 


137 


thrown  off.  Hold  the  feather  so  as  to  separate  one-third  of 
the  string  and  cause  it  to  vibrate ;  the  remainder  of  the  cord 
will  vibrate  in  two  segments.  When  the  feather  is  removed, 


FIG.  128. 


FIG.  129. 


the  entire  cord  will  vibrate  in  three  different  parts  of  equal 
length,  separated  by  stationary  points  called  nodes.  This 
may  be  shown  by  the  riders  ;  the  one  at  the  node  remains, 
while  the  others  are  thrown  off. 

(4.)  ACOUSTIC  FIGURES. — Sprinkle  fine  sand  on  a  metal 
plate.  Place  the 
finger-nail  on  one 
edge  to  stop  the 
vibration  at  that 
point,  as  the 
feather  did  in  the 
last  experiment, 
and  draw  the  bow 
lightly  across  the 
opposite  edge. 
The  sand  will  be 
tossed  away  from 

the  vibrating  parts  of  the  plate  and  will 
collect  along  the  nodal  lines,  which  divide 
the  large  square.     It  is  wonderful  to  see  how  the  sand  will 
seemingly  start  into  life  and  dance  into  line  at  the  touch  of 


138 


ACOUSTICS. 


the  bow.  Fig.  130 
shows  some  of  the 
beautiful  patterns  ob- 
tained by  Chladni. 

(5.)  HARMONICS.* 
— Whenever  a  cord 
vibrates,  it  separates 
into  segments  at  the 
same  time.-  Thus  we 
have  the  full  or  fun- 
damental note  of  the 
entire  string,  and  su- 
perposed upon  it  the 
higher  notes  produced 
by  the  vibrating  parts. 
These  are  called  overtones  or  "harmonics.  The  mingling  of 
the  two  classes  of  vibrations  determines  the  quality  of  the 
sound,  and  enables  us  to  distinguish  the  music  of  different 
instruments. 

(6.)  NODES  OF  A  BELL. — Let 
the  heavy  circle  in  Fig.  131  repre- 
sent the  circumference  of  a  bell 
when  at  rest.  Let  the  hammer 
strike  at  #,  Z>,  c,  or  d.  At  one 
moment,  as  the  bell  vibrates,  it 
forms  an  oval  with  ab,  at  the 
next  with  cd,  for  its  longest  diam- 
eter. When  it  strikes  its  deepest 
note,  the  bell  vibrates  in  four 
segments,  with  n,  n}  n,  n,  as  the 

*  Press  gently  but  firmly  down  the  notes  C,  G,  and  C,  in  the  octave  above  middle 
C,  on  the  piano-forte.  Without  releasing  these  keys,  give  to  C  below  middle  C  a 
quick,  hard  blow.  The  damper  will  fall,  and  the  sound  will  stop  abruptly.  At  the 
same  instant  a  low,  soft  cnord  will  be  heard.  This  comes  from  the  three  strings 
whose  dampers  are  raised,  leaving  them  free  to  sound  in  sympathy  with  the  over- 
tones of  the  lower  C,  w'aich  sounds  are  identical  with  their  own.— When  a  goblet  or 
wine-glass  is  tapped  with  a  knife-blade,  we  can  distinguish  three  sounds,  the  fun- 
damental and  two  harmonics. 


VIBRATIONS    OF    CORDS.  139 

nodal  points,  whence  nodal  lines  mn  up  from  the  edge  to 
the  crown  of  the  bell.  It  tends,  however,  to  divide  into  a 
greater  number  of  segments,  especially  if  it  is  very  thin, 
and  to  produce  harmonics.  The  overtones  which  follow 
the  deep  tones  of  the  bell  are  frequently  very  striking,  even 
in  a  common  call-belL 

(7.)  NODES  OF  A  SOUKDIETG-BOARD. — The  case  of  a  violii 
or  guitar  is  composed  of  thin  wooden  plates  which  divide 
into  vibrating  segments,  separated  by  nodal  lines  according 
to  the  pitch  of  the  note  played.  The  enclosed  air  vibrating 
in  unison  with  these,  reinforces  the  sound  and  gives  il 
fullness  and  richness. 

(8.)  MUSICAL  SCALE. — The  tone  produced  by  an  entire 
string  is  called  its  fundamental  sound.  The  notes  of  the 
scale  above  this  are  given  by  the  parts  of  the  string  indi- 
cated by  the  following  fractions  : 

C,        D,        E,        F,        G,        A,        B,        C. 

iff!  t  t  A  i 
As  the  number  of  vibrations  varies  inversely  as  the 
length  of  the  cord,  we  need  only  to  invert  these  fractions 
to  obtain  the  relative  number  of  vibrations  per  second ; 
thus,  -§,  f ,  f,  |,  -J,  ^-,  2.  Eeduced  to  a  common  denom- 
inator, their  numerators  are  proportional,  and  we  have  the 
whole  numbers  which  represent  the  relative  rates  of  vibra- 
tion of  the  notes  of  the  scale,  viz. : 

24,    27,    30,    32,    36,    40,    45,    48. 

The  number  of  vibrations  corresponding  to  the  different 
letters  is,* 

C,  D,  E,  F,  G,  A,  B,         1C. 

128,        144,         160,         170,        192,         214,         240,         256. 

*  In  this  table,  "  C  =  256  vibrations  "  represents  the  middle  C  of  a  piano-forte. 
This  number  is  purely  arbitrary.  The  so-called  "  concert-pitch  "  varies  in  different 
countries.  The  Stuttgart  Congress  of  1834  fixed  the  standard  tuning-fork—middle 
A— at  440  vibrations  per  second,  which  would  make  middle  C  =  264 ;  while  the  Paris 
Conservatory  (1859)  gave  to  middle  A  437.5,  and  to  middle  C  261.  The  English 
tuning-fork  represents  C  in  the  treble  staff,  and  makes  528  vibrations,  the  pitch  being 
the  same  as  the  Stuttgart.  The  ratio  of  the  different  letters  is  identical,  whatever 
the  pitch. 


140 


ACOUSTICS. 


FIG.  132. 


8.  "Wind  Instruments  produce  musical  sounds  by 
enclosed  columns  of  air.  Sound-waves  run  backward  and 
forward  through  the  tube  and  act  on  the  surrounding  air 
like  the  vibrations  of  a  cord.  The  sound-waves  in  organ- 
pipes  are  set  in  motion  by  either  fixed  mouth- 
pieces or  vibrating  reeds.  The  air  is  forced 
from  the  bellows  into  the  tube  P,  through 
the  vent  i,  and  striking  against  the  thin  edge 
a,  produces  a  nutter.  The  column  of  air 
above,  thrown  into  vibration,  reinforces  the 
sound  and  gives  a  full  musical  tone.  The 
length  of  the  pipe  determines  the  pitch.  The 
variation  in  the  quality  of  different  wind  in- 
struments is  caused  by  the  mingling  of  the 
harmonics  with  the  fundamental  tone.  In 
the  flute,  for  example,  the  vibrating  column 
of  air  may  be  broken  up  into  segments  by 
varying  the  force  of  the  breath. 

9.  Sympathetic  Vibrations,  or  Res- 
onance.— Produce  a  musical  tone  with  the 
voice  near  a  piano,  and  a  certain  wire  will 
select  that  sound  and  respond  to  it.  Change 
the  pitch,  and  the  first  string  will  cease, 
while  another  replies.  If  a  hundred  tuning- 
forks  of  different  tones  are  sounding  at  the 
foot  of  an  organ-pipe,  it  will  choose  the  one 
to  which  it  can  reply,  and  answer  that 
alone.*  Helmholtz  has  applied  this  principle  to  the  con- 
struction of  the  resonance  globe,  an  instrument  which  will 
respond  to  a  particular  harmonic  in  a  compound  tone,  and 
strengthen  it  so  as  to  make  it  audible. 

(1.)  SENSITIVE  FLAMES. — Flames  are  sensitive  to  sound. 
At  an  instrumental  concert  the  gas-lights  vibrate  with  cer- 

*  Two  clocks  set  on  one  shelf  or  against  the  same  wall,  affect  each  other.— 
Watches  in  the  shop-window  keep  better  time  than  when  carried  singly.- 


THE    PHOKOGRAPH. 


141 


FIG.  133. 


tain  pulsations  of  the  music.  This  is  noticeable  when  the 
pressure  of  gas  is  so  great  that  the  flame  is  just  on  the 
verge  of  flaring,  and  the  vibration  of  the  sound-wave  is 
sufficient  to  "push  it  over  the 
precipice.  "* 

(2.)  SINGING  FLAMES. — If 
we  lower  a  glass  tube  over  a 
small  gas-jet,  we  soon  reach  a 
point  where  the  flame  leaps 
spontaneously  into  song.  At 
first  the  sound  seems  far  re- 
mote, but  gradually  approaches 
until  it  bursts  into  an  almost 
intolerable  scream.  The  length 
of  the  tube  and  the  size  of  the 
jet  determine  the  pitch  of  the 
note,  f  The  flame,  owing  to  the 
friction  at  the  mouth  of  the 
pipe,  is  thrown  into  vibration. 
The  air  in  the  tube,  being 
heated,  rises,  and  not  only  vi- 
brates in  unison  with  the  jet, 
but,  like  the  organ-pipe,  selects 
the  tone  corresponding  to  its 
length. 

1C.    The   Phonograph  is 

an  instrument   for   recording 

the  sound  vibrations.  It  consists  of  (1)  an  outer  tube  (or 
ear)  for  receiving  the  voice  vibrations ;  (2)  at  the  bottom 
of  this  a  thin  plate  (or  membrane)  which  vibrates  in  uni- 
son with  the  voice ;  (3)  at  the  back  of  the  membrane  a 

*  Prof.  Barrett,  of  Dublin,  describes  a  peculiar  jet  which  is  so  sensitive  that  it 
trembles  and  cowers  at  a  hiss,  like  a  human  being,  beats  time  to  the  ticking  of  a 
watch,  and  is  violently  agitated  by  the  rumpling  of  a  silk  dress. 

t  See  Chemistry,  p.  55.  The  jets  are  easily  made  by  drawing  out  glass  tubing  to  a 
fine  point  over  a  spirit-lamp.  The  length  of  the  tube  may  be  varied,  as  in  £ie  figure, 
by  a  paper  tube, «. 

DIVERSITY  I 


ACOUSTICS. 


FIG.  134. 


lever  which  is  moved  by  these  vibrations ;  (4)  at  the  end 
of  the  lever  a  sharp  point,  which  traces  on  a  sheet  of  tin- 
foil marks  corresponding  to  these  vibrations;  (5)  a  cylinder 
wound  with  a  sheet  of  the  foil,  and  made,  by  clockwork, 
to  revolve  slowly  under  the  pen-point. 

After  the  voice  has  thus  engraved  on  the  foil  its  vibra- 
tions, the  cylinder  can  be  reset  and  the  point  following 
the  indentations  on  the  foil  will  move  the  lever,  strike 
the  membrane,  and  reproduce  through  an  outer  tube  (or 
trumpet)  the  original  sound.  The  sheets  of  foil  may  be 
taken  from  the  cylinder  and  kept  for  any  length  of  time, 
to  be  used  when  wanted.  . 

11.  The  Ear.— In  Fig.  134,  the  ear  is  represented  (Hy- 
gienic Physiology,  p.  216).  The  sound-wave  passes  into  the 

auditory  canal,  B, 
which  is  about  an  inch 
in  length,  and  strik- 
ing against  the  mem- 
brane of  the  tympa- 
num or  drum,  which 
closes  the  orifice  of  the 
external  ear,  throws  it 
into  vibration.  Next, 
the  series  of  small 
bones,  0,  called  re- 
spectively, from  their 
peculiar  form,  the 
hammer,,  anvil,  and 
stirrup,  conduct  the 
motion  to  the  inner  ear,  which  is  termed,  from  its 
complicated  structure,  the  labyrinth.  This  is  filled  with 
liquid,  and  contains  the  semi-circular  canals,  D,  and  the 
cochlea  (snail-shell),  E,  which  receive  the  vibrations  and 
transmit  them  to  the  auditory  nerve,  the  fine  filaments  of 
which  are  spread  out  to  catch  every  pulsation  of  the  sound- 
wave. The  middle  ear,  which  contains  the  chain  of  small 


THE    EAK.  143 

bones,  is  a  cavity  about  half  ai*  inch  in  diameter,  filled 
with  air,  communicating  with  the  mouth  by  the  EustacMan 
tube,  G.*  Within  the  labyrinth  are  fine,  elastic  hair-bristles 
and  crystalline  particles  among  the  nerve-fibres,  wonderfully 
fitted,  the  one  to  receive  and  the  other  to  prolong  the  vibra- 
tions ;  and  lastly,  a  lute  of  3,000  microscopic  strings,  so 
stretched  as  to  vibrate  in  unison  with  any  sound. 

(1.)  KANGE  OF  THE  EAR. — Helmholtz  fixes  the  highest 
limit  of  musical  sounds  at  38^000  vibrations  per  second,  and 
the  lowest  at  16.  f  Below  this  number  the  pulses  cease 
to  link  themselves,  and  become  distinct  sounds.};  The 
range  of  the  ear  is  thus  about  eleven  octaves.  The  capacity 
to  hear  the  higher  tones  varies  in  different  persons.  A 
sound  audible  to  one  may  be  silence  to  another.  Some 
ears  cannot  distinguish  the  squeak  of  a  bat  or  the  chirp 
of  a  cricket,  while  others  are  acutely  sensitive  to  these  shrill 
sounds.  Indeed,  the  auditory  nerve  seems  generally  more 
alive  to  the  short,  quick  vibrations  than  to  the  long,  slow 
ones.  The  whirr  of  a  locust  is  much  more  noticeable  than 
the  sighing  of  the  wind  through  the  trees.  § 


*  The  Eustachian  tube  serves  to  connect  the  tnner  cavity  with  the  external  atmos- 
phere. If  at  any  time  the  pressure  of  the  air  without  becomes  greater  or  less  than 
that  within,  the  membrane  of  the  tympanum  feels  the  strain,  pain  is  experienced, 
and  partial  deafness  ensues.  A  forcible  concussion  frequently  produces  this  result. 
In  the  act  of  swallowing,  the  tube  is  opened  and  the  equilibrium  restored.  We  may 
force  air  Into  the  cavity  of  the  ear  by  closing  the  mouth  and  nose,  and  forcibly 
expiring  the  air  from  the  lungs.  This  will  render  us  insensible  to  low  sounds,  as  the 
rumble  of  a  railway-train,  while  we  can  hear  the  higher  ones  as  usual. 

t  A  tone  produced  by  about  16  vibrations  per  second  may  be  made  by  inserting 
the  finger  lightly  in  the  ear,  bringing  at  the  same  time  the  muscles  of  the  hand  into 
strong  contraction.  A  sound  will  be  heard  which  is  as  deep  as  the  toll  of  a  cathedral 
bell. 

$  Our  unconsciousness  Is  no  proof  of  the  absence  of  sound.  There  are,  doubt- 
less, sounds  in  Nature  of  which  we  have  no  conception.  Could  our  sense  be  quick- 
ened, what  celestial  harmony  might  thrill  us  I  Prof.  Cooke  beautifully  says  :  "  The 
very  air  around  us  may  be  resounding  with  the  hallelujahs  of  the  heavenly  host ; 
While  our  dull  ears  hear  nothing  but  the  feeble  accents  of  our  broken  prayers." 

§  To  this,  however,  there  are  remarkable  exceptions.  The  author  knows  a  lady 
who  is  insensible  to  the  higher  tones  of  the  voice,  but  acutely  sensitive  to  the  lower 
ones.  Thus,  on  one  occasion,  being  in  a  distant  room,  she  did  not  notice  the  ringing 
of  the  bell  announcing  dinner,  but  heard  the  noise  the  bell  made  when  returned  to 
Ub  plucu  ou  Liic  shelf. 


144  ACOUSTICS. 

(2.)    THE  ABILITY  OF  THE  EAR  TO  DETECT  AND  ANALYZE 

SOUND  is  wonderful  beyond  comprehension.  Sound-waves 
chase  one  another  up  and  down  through  the  air,  super- 
posed in  entangled  pulsations,  yet  a  cylinder  not  larger 
than  a  quill  conveys  them  to  the  ear,  and  each  string  of 
that  wonderful  harp  selects  its  appropriate  sound,  and  re- 
peats the  music  to  the  soul  within.  Though  a  thousand 
instruments  be  played  at  once,  there  is  no  confusion,  but 
each  is  heard,  and  all  blend  in  harmony.* 

PRACTICAL  QUESTIONS.— 1.  Why  cannot  the  rear  of  a  long  column  of  soldiers 
keep  time  to  the  music  in  front  ?  2.  Three  minutes  elapse  between  the  flash  and  the 
report  of  a  thunderbolt ;  how  for  distant  is  it  ?  3.  Five  seconds  expire  between  the 
flash  and  the  report  of  a  gun ;  what  is  its  distance  7  4.  Suppose  a  speaking-tube 
should  connect  two  villages  ten  miles  apart ;  how  long  would  it  take  the  sound  to 
travel  ?  5.  The  report  of  a  pistol-shot  was  returned  to  the  ear  from  the  face  of  a 
cliff  in  four  seconds ;  what  was  the  distance  ?  6.  What  is  the  cause  of  the  difference 
between  the  voice  of  man  and  woman  ?  A  base  and  a  tenor  voice  ?  7.  What  is  the 
number  of  vibrations  per  second  necessary  to  produce  the  fifth  tone  of  the  scale  oi 
C  ?  8l What  is  the  length  of  each  sound-wave  in  that  tone  when  the  temperature  is 
at  zero  r  9.  What  is  the  number  of  vibrations  in  the  fourth  tone  above  middle  C  ? 
10.  A  meteor  of  Nov.  13, 1868,  exploded  at  a  height  of  60  miles ;  what  time  was  needed 
for  its  sound  to  reach  the  earth  ?  ll^j  A  stone  is  let  fall  into  a  well,  and  in  four 
seconds  is  heard  to  strike  the  bottom ;  how  deep  is  the  well  ?  ;  12.  What  time  would 
be  required  for  a  sound  to  travel  five  miles  in  the  still  wafer  of  a  lake  ?  13.  How 
much  louder  will  be  the  report  of  a  gun  to  an  observer  at  a  distance  of  20  rods  than 
to  one  at  half  a  mile  ?  14.  Does  sound  travel  faster  at  the  foot  than  at  the  top  of  a 
mountain  ?  15.  Why  is  an  echo  weaker  than  the  original  sound  ?  16.  Why  is  it  so 
fatiguing  to  talk  through  a  speaking-trumpet  ?  17.  Why  will  the  report  of  a  cannon 
fired  in  a  valley  be  heard  on  the  top  of  a  neighboring  mountain,  better  than  one  fired 
on  the  top  of  a  mountain  will  be  heard  in  the  valley?  18.  Why  do  our  footsteps  in 
unfurnished  dwellings  sound  so  startlingly  distinct  ?  19.  Why  do  the  echoes  of  an 
empty  church  disappear  when  the  audience  assemble  ?  20.  What  is  the  object  of  the 
sounding-board  of  a  piano  ?  21.  During  some  experiments,  Tyndall  found  that  a 
certain  sound  would  pass  through  twelve  folds  of  a  dry  silk  handkerchief,  but  would 
be  stopped  by  a  single  fold  of  a  wet  one.  Explain.  22.  What  is  the  cause  of  the 
musical  murmur  often  heard  near  telegraph  lines  ?  23.  Why  will  a  variation  in  the 
quantity  of  water  in  the  goblet,  when  made  to  sound,  in  the  experiment  described 
on  p.  123,  cause  a  difference  in  the  tone  ?  24.  At  what  rate  (in  metres)  will  sound 
move  through  air,  the  temperature  being  20°  C.  t 

*  "  Is  not  the  ear  the  most  perfect  sense  ?  A  needle  woman  will  distinguish  by  the 
sound  whether  it  is  silk  or  cotton  that  is  torn.  Blind  people  recognize  the  age 
of  persons  by  their  voices.  An  architect,  comparing  the  length  of  two  lines  sepa- 
rated from  each  other,  if  he  estimate  within  &,  we  deem  very  accurate ;  but  a 
musician  would  not  be  considered  very  precise  who  estimated  within  a  quarter  of  a 
note  (128  -«-  30  =  4  nearly).  In  a  large  orchestra,  the  leader  will  distinguish  each  note 
of  each  instrument.  We  recognize  an  old-time  friend  by  the  sound  of  his  voice,  when 
the  other  senses  utterly  fail  to  recall  him.  The  musician  carries  in  his  ear  the  idea 
of  the  musical  key  and  every  tone  in  the  scale,  though  he  is  constantly  hearing  a 
multitude  of  sounds.  A  tune  once  learned,  will  be  remembered  when  the  words  of 
the  song  are  forgotten." 


SUMMARY.  145 


SUMMARY. 

Sound  is  produced  by  vibrations.  These  are  transmitted  in  waves 
through  the  air  (60°  F.)  at  the  rate  of  1120  feet  per  second ;  through 
water  four  times,  and  through  iron  fifteen  times  as  fast.  In  gen- 
eral, the  velocity  depends  on  the  relation  between  the  density  and  the 
elasticity  of  the  medium;  and  the  intensity  is  proportional  to  the 
square  of  the  amplitude  of  the  molecular  vibrations.  Sound,  like 
light,  may  be  reflected  and  refracted  to  a  focus.  Echoes  *  are  produced 
by  the  reflection  of  sound  from  smooth  surfaces,  not  less  than  112  feet 
(about  33  metres)  distant.  Rapidly-repeated  vibrations  make  a  contin- 
uous sound ;  regular  and  rapid  vibrations  produce  music ;  irregular 
ones  cause  a  noise. 

'S  The  pitch  of  a  sound  .depends  on  the  rapidity  of  the  vibrations. 
The  number  of  waves,  and  their  consequent  length  in  a  given 
sound,  is  found  by  means  of  the  siren.  Unison  is  produced  by 
identical  wave-motions.  Any  number  of  sound-waves  may  traverse  the 
air,  as  any  number  of  water-waves  may  the  surface  of  the  sea,  without 
losing  their  individuality.  The  motion  of  each  molecule  of  air  is  the 
algebraic  sum  of  the  several  motions  it  receives.  Two  systems  of 
waves  may  therefore  destroy  or  strengthen  each  other,  according  as 
their  several  condensations  or  rarefactions  coalesce.  Interference  is 
the  mutual  destruction  of  two  systems  of  waves.  "  Beats  "  is  the  effect 
produced  by  two  musical  sounds  of  nearly  the  same  pitch,  which 
alternately  interfere  and  coincide.  The  vibrations  of  a  cord  .produce 
a  musical  sound,  which  is  reinforced  by  a  sounding-board.  The  rate 
of  vibration  and  consequent  pitch  depends  on  the  length,  the  tension, 
and  the  weight  of  the  cord.  Sounding  bodies  tend  to  vibrate  in  seg- 
ments. The  harmonics  thus  produced  give  the  quality  (timbre)  of 
different  sounds.  The  various  notes  in  the  musical  scale  are  deter- 

*  Several  acoustic  phenomena  have  become  of  historical  interest.  (1.)  Near 
Syracuse,  Sicily,  is  a  cave  known  as  the  Ear  of  Dionysius.  A  whisper  at  the  farther 
end  of  the  cavern  is  easily  heard  by  a  person  at  the  entrance,  though  the  distance  is 
200  feet.  Tradition  says  that  the  Tyrant  of  Syracuse  used  this  as  a  dungeon,  and 
was  thus  enabled  to  listen  to  the  conversation  of  his  unfortunate  prisoners.  (2.) 
On  the  banks  of  the  Nile,  near  Thebes,  is  a  statue  47  feet  high,  and  extending  7  feet 
below  the  ground.  It  is  called  the  Vocal  Memnon.  Ancient  writers  tell  us  that 
about  sunrise  each  morning,  there  issued  from  this  gigantic  monolith  a  musical 
sound  resembling  the  breaking  of  a  harp-string.  It  is  now  believed  that  this  was 
produced  by  strong  currents  of  air  (due  to  the  change  of  temperature  in  the  early 
morning)  passing  through  crevices  in  the  stone.  (3.)  Near  Mount  Sinai,  in  Arabia, 
remarkable  sounds  are  produced  by  the  sand  falling  down  a  declivity.  The  sand, 
which  is  very  white,  fine  and  dry,  lies  at  such  an  angle  as  to  be  easily  set  in  motion 
by  any  cause,  such  as  scraping  away  a  little  at  the  foot  of  the  slope.  The  sand  then 
rolls  down  with  a  sluggish  motion,  causing  at  first  a  low  moan,  that  gradually  swells 
to  a  roar  like  thunder,  and  finally  dies  away  as  the  motion  ceases. 


146  '  ACOUSTICS. 

mined  by  fixed  portions  of  the  length  of  the  cord.  The  music  of  a 
wind-instrument  is  produced  by  vibrating  columns  of  air.  Resonance 
is  a  sympathetic  vibration  caused  by  one  sonorous  body  in  another,  as 
seen  in  sensitive  flames,  the  resonance  globe,  etc.  The  voice  is  a  reed 
instrument,  with  its  vibrating  cords  and  resonant  cavity.  The  ear 
collects  the  sound-waves  and  transmits  the  motion  to  the  brain.  It 
consists  of  the  outer  ear,  the  drum  and  the  labyrinth. 


HISTORICAL    SKETCH. 

The  ancients  knew  that  without  air  we  should  be  plunged  in  eternal 
silence.  "What  is  the  sound  of  the  voice,"  cried  Seneca,  "but  the 
concussion  of  the  air  by  the  shock  of  the  tongue  ?  What  sound  could 
be  heard  except  by  the  elasticity  of  the  aerial  fluid  ?  The  noise  of 
horns,  trumpets,  hydraulic  organs,  is  not  that  explained  by  the  elastic 
force  of  the  air  ?  "  Pythagoras,  who  lived  in  the  6th  century  before 
Christ,  conceived  that  the  celestial  spheres  are  separated  from  each 
other  by  intervals  corresponding  with  the  relative  lengths  of  strings 
arranged  to  produce  harmonious  tones.  In  his  musical  investigations 
he  used  a  monochord,  the  original  of  the  sonometer  now  employed  by 
physicists,  and  wished  that  instrument  to  be  engraved  on  his  comb. 
Pythagoras  held  that  the  musical  intervals  depend  on  mathematics ; 
while  his  great  rival,  Aristoxenes,  claimed  that  they  should  be  tested 
by  the  ear  alone.  The  theories  of  these  two  philosophers  long  divided 
the  attention  of  the  scientific  world. 

Many  centuries  elapsed  before  any  marked  advance  was  made. 
Galileo  called  attention  to  the  sonorous  waves  traversing  the  surface 
of  a  glass  of  water,  when  the  glass  is  made  to  vibrate.  Newton 
believed  sound  to  be  transmitted  by  aerial  waves,  and  estimated  the  rate. 

The  present  century  has  witnessed  a  more  complete  demonstration 
of  the  laws  of  the  vibrations  of  cords  and  the  general  principles  of 
sound.  In  1822,  Arago,  Gay-Lussac  and  others  decided  the  velocity 
of  sound  to  be  337  metres  at  10°  C.  Savart  invented  a  toothed  wheel 
by  which  he  determined  the  number  of  vibrations  in  a  given  sound  ; 
Latour  discovered  the  siren,  which  gave  still  more  accurate  results ; 
Colladon  and  Sturm,  by  a  series  of  experiments  at  Lake  Geneva,  found 
the  velocity  of  sound  in  water ;  Helmholtz  made  known  the  laws  of 
harmonics  ;  Lissajous,  by  means  of  a  mirror  attached  to  the  vibrating 
body,  threw  the  vibrations  on  a  screen  in  a  series  of  curves,  and  so 
rendered  them  visible ;  while  Tyndall  has  investigated  the  causes 
modifying  the  propagation  of  sound,  as  acoustic  clouds,  fogs,  etc.* 
and  poj  ularized  the  whole  subject  of  acoustics.  (References,  p.  120.) 


VII. 

O  JV     L  I  G-  H  T. 


TIu  sunbeam  comes  to  the  earth  as  simply  motion  of  ether-waves,  yet  it  is 
the  grand  source  of  beauty  and  power.  Its  heat,  light,  and  chemical  force 
work  everywhere  the  miracle  of  life  and  motion.  In  the  growing  plant, 
the  burning  coal,  the  flying  bird,  the  glaring  lightning,  the  blooming  flower, 
the  rushing  engine,  the  roaring  cataract,  the  pattering  rain — we  see  only 
itvritj  manifestations  of  this  one  all-energizing  force. 


ANALYSIS. 


'on: 


1.  DEFINITIONS. 

2.  VISUAL  ANGLE, 

3.  LAWS  OF  LIGHT. 

4.  VELOCITY  OF  LIGHT. 

5.  THEORY  OF  LIGHT. 


ri. 

DEFINITION  AND  LAW. 

j_! 

3. 

ACTION  OF  ROUGH  AND  POLISHED  SURFACES. 

I 

g 

i(a.)  Effect  of. 

(b.)  Image  seen. 

u. 
o 

(c.)  Image  behind 
mirror. 

z 

(d.)  Multiple  images. 

O 

8. 

(e.)  Images  in  water. 

h- 

O 

LU 

(2.)  Con-   ((a.)  J^srf  o/. 
cave.     |  (b.)  Image  seen. 

tr 

(3.)  Con-   ((a.)  Effect  of  . 
vex.       j  (b.)  Image  seen. 

k  4. 

TOTAL  REFLECTION. 

•^ 

f  i 

DEFINITION  AND  ILLUSTRATIONS. 

02 

OfJ 

2'. 

LAWS  OF  REFRACTION  AND  ILLUSTRATIONS. 

0 

M 

I-I 
og 

f  (1.)  Con-  (  (a.)  Effect  of. 

E< 

Ll_  .^ 

3. 

LENSES      <          vex<       <  ^^  /ma^e  seen' 
ES'      ]  (2.)  Con-  ]  (a.)  Effect  of  . 

0 

So 

(.         cave.     1  (b.)  Image  seen. 

4. 

ABERRATION. 

CO 

I5- 

MIRAGE. 

'  j< 

SOLAR  SPECTRUM. 

Lu 

2. 

THREE  CLASSES  OF  RAYS. 

o 

3. 

THREE  KINDS  OF  SPECTRA. 

z 

4. 

THE  SPECTROSCOPE. 

0    . 

C  (1.)  Formation  of. 

-53; 

•RATxrorvcir   J   (2.)  Primary  Bow. 

o  S2 

. 

)W'    1   (3.)  Secondary  Bow. 

Q__J 

I  (4.)  Why  the  Bow  is  Circular, 

0 

6. 

COMPLEMENTARY  COLORS. 

0 

7. 

INTERFERENCE  OF  LIGHT. 

. 

8. 

COLOR. 

„  9. 

POLARIZATION  OF  LIGHT. 

1.  MICROSCOPE. 

2.  TELESCOPE. 

3.  OPERA  GLASS. 

4.  STEREOSCOPE. 

5.  MAGIC  LANTERN. 

6.  CAMERA. 

7.  EYE. 


OPTICS,     OR    THE     SCIENCE 
OF     LIGHT. 

1.    PRODUCTION    AND    TRANSMISSION    OF    LIGHT. 

1.  Definitions. — A  luminous  body  is  one  that  emits 
light.  A  medium  is  any  substance  through  which  light 
passes.  A  transparent  *  body  is  one  that  obstructs  light  so 
little  that  we  can  see  objects  through  it.  A  translucent  body 
is  one  that  lets  some  light  pass,  but  not  enough  to  render 
objects  visible  through  it.  An  opaque  body  is  one  that  does 
not  transmit  light.  A._ray  of  light  is  a  single  line  of  light ; 
it  may  be  traced  in  a  dark  room  into  which  a  sunbeam  is 
admitted  by  the  floating  particles  of  dust  which  reflect  the 
light  to  the  eye.  K  pencil  or  learn  of  liglit  is  a  collection  of 
rays,  which  may  be  parallel,  diverging  or  converging. 

2.  The  Visual  Angle  is  the  angle  formed  at  the  eye 
by  rays  coming  from  the  extremities  of  an  object.  The 
angle  AOB  is  the  angle  of  vision  subtended  by  the  object 

FIG.  185. 


AB.  The  size  of  this  angle  varies  with  the  distance  of  the 
body.  AB  and  A'B'  are  of  the  same  length,  and  yet  the 
angle  A'OB'  is  smaller  than  AOB,  and  hence  A'B'  will  seem 

*  The  terms  transparent  and  opaque  are  relative.  No  substance  is  perfectly 
transparent,  or  entirely  opaque.  Glass  obstructs  some  light.  According  to  Miller 
7  feet  of  the  clearest  water  will  arrest  one-half  the  light  which  fall?  upon  it.  While 
Young  asserts  that  the  beam  of  the  setting  sun,  passing  through  200  miles  of  air,  loses 
&§g  of  its  force.  On  the  other  hand,  gold,  beaten  into  leaf,  becomes  translucent,  and 
of  a  feint  green  color;  and  scraped  horn  is  semi-transparent 


150  OPTICS. 

shorter  than  AB.  The  distance  and  the  size  01  objects  are 
intimately  connected,  since  by  experience  we  have  learned 
to  associate  them.  Knowing  the  distance  of  an  object,  we 
immediately  determine  its  size  from  the  visual  angle.* 

3.  Laws  of  Light. — I.  Light  passes  off  from  a  lumi- 
nous body  equally  in  every  direction.      II.    Light  travels 
through  a  uniform  medium   in  straight  lines.      III.  The 
intensity  of  light  decreases  as  the  square  of  the  distance 

V    increases. 

4.  The  Velocity  of  Light  has  been  determined  in 
various  ways.     The  following  was  the  first  method :  The 
planet  Jupiter  has  four  moons.     As  these  revolve  around 
the  planet,  they  are  eclipsed  at  regular  intervals.     In  the 
cut,  let  J  represent  Jupiter,  e  one  of  the  moons,  S  the  sun, 

FIG.  136. 


and  T  and  t  different  positions  of  the  earth  in  its  orbit 
around  the  sun.  When  the  earth  is  at  T,  the  eclipse  occurs 
16  min.  and  36  sec.  earlier  than  at  t.  That  interval  of  time 
is  required  for  the  light  to  travel  across  the  earth's  orbit, 
giving  a  velocity  of  about  186,000  miles  per  second,  f 

5.  TJncUilatory  Theory  of  Light. — There  is  supposed 
to  be  a  fluid,  termed  ether,  constituting  a  kind  of  universal 
atmosphere,  diffused  through  space.  It  is  so  subtle  that  it 

*  We  can  vary  the  apparent  size  of  any  body  at  which  we  ars  looking  by  increasing 
or  diminishing  this  angle — a  principle  that  will  be  found  of  great  importance  in  the 
formation  of  images  by  mirrors  and  lenses. 

.  t  This  rate  is  so  great  that  for  all  distances  on  the  earth  it  is  instantaneous.    A 
sunbeam  would  girt  the  globe  quicker  than  we  can  wink. 


REFLECTION    OF    LIGHT.  15-1 

glides  among  the  molecules  of  bodies  as  the  air  does  among 
the  branches  and  the  foliage  of  trees.  It  fills  the  pores  of  all 
substances,  eludes  all  chemical  tests,  passes  in  through  the 
receiver,  and  remains -even  in  the  vacuum  of  an  air-pump. 
A  luminous  body  sets  in  motion  waves  of  ether,  which  go 
off  in  every  direction.  They  move  at  the  rate  of  186,000 
miles  per  second,  and  breaking  upon  the  eye,  give  the  im- 
pression of  sight.  The  wave-motion  is  like  that  of  sound, 
except  that  the  vibrations  are  transverse  (crosswise).* 

2.    REFLECTION    OF    LIGHT. 

1.  Definition. — Light  falling  on  a  surface  is  divided 
into  two  portions.     One  enters  the  body ;  the  other  is  re- 
flected f  according  to  the  familiar  law  of  Motion  and  of 
Sound  :  The  angle  of  incidence  =  that  of  reflection. 

2.  Action  of  Bough  and  Polished  Surfaces.— When 
the  surface  is  rough,  the  numerous  little  elevations  scat-  j, 
ter  the  reflected  rays  in  every  direction,  forming  diffused   [, 
light.     Such  a  body  can  be  seen  from  any  point.     "When  the  Jl 
surface  is  polished,   the    rays  are  uniformly  reflected  in 
particular  directions,  and  bring  to  us  the  images  of  other 
objects.     We  thus  see  non-lurninous  objects  by  irregularly- 
reflected  (diffused)  light,  and  images  of  objects  by  regularly- 
reflected  light.  J 

3.  Mirrors. — All  highly-reflecting  surfaces  are  mirrors. 
These  are  of  three  kinds — plane,  concave  and  convex.     The     \! 

*  Thus,  if  we  suppose  a  star  directly  overhead  and  a  ray  of  light  coming  down  to 
us,  we  should  conceive  that  the  particles  which  compose  the  waves  are  vibrating 
N.  S.  E.  W.,  and  toward  every  other  point  of  the  compass  all  at  once. 

t  The  amount  of  light  reflected  varies  with  the  angle  at  which  light  falls.  Thus, 
if  we  look  at  the  images  of  objects  in  etill  water,  we  notice  that  those  near  us  are  not 
as  distinct  as  those  on  the  opposite  bank.  The  rays  from  the  latter  striking  the 
water  more  obliquely  are  more  perfectly  reflected  to  the  eye.— Fill  a  sheet-iron  or  any 
dark-colored  pail  with  water  tinted  with  bluing  or  red  ink.  The  color  will  be  quite 
invisible  to  a  spectator  at  a  little  distance.  Now  insert  in  the  water  a  plate.  This 
will  reflect  the  transmitted  light  and  reveal  the  hue  of  the  water. 

J  The  most  perfectly  polished  substance,  however,  diffuses  some  light— enough  to 
enable  us  to  trace  its  surface ;  were  it  not  so,  we  should  not  be  aware  of  its  exist, 
ence.  The  deception  of  a  large  plate-glass  mirror  is  often  nearly  complete  ;  but  dust 
or  vapor,  increasing  the  irregular  reflection,  will  bring  its  surface  to  view. 


152 


ornos. 


FIG.  187. 


first  has  a  flat  surface  ;  the  second,  one  like  the  inside,  and 
the  third,  one  like  the  outside  of  a  watch-crystal.  The 
general  principle  of  mirrors  is  that  the  image  is  s-een  in  the 
direction  of  the  reflected  ray  as  it  enters  the  eye. 

(1.)  PLANE  MIBKOBS. — Kays  of  light  retain  their  relative 
direction  after  reflection  from  a  plane  surface.*  An  image 
seen  in  a  plane  mirror  is  therefore  erect  and  of  the  same  size 
as  the  object.  It  is,  however,  reversed  right  and  left. 

Why  the  image  is  as  far  behind  the  mirror  as  the  object  is 
in  front.  Let  AB  be  an  arrow  held  in  front  of  the  mirror 

MN.  Rays  of  light  from 
the  point  A  striking  upon 
the  mirror  at  C,  are  reflect- 
ed, and  enter  the  eye  as  if 
they  came  from  a.  Eays 
from  B  seem  to  come  from 
T).  Since  the  image  is  seen 
in  the  direction  of  the  re- 
flected rays,  it  appears  at 
ab,  a  point  which  can  easily 
be  proved  to  be  as  far  be- 
hind MN  as  the  arrow  is  in  front  of  it.  Such  an  image 
is  called  a  virtual  one,  as  it  has  no  real  existence. 

Why  ive  can  see  several  images  of  an 
object  in  a  mirror.  Metallic  mirrors 
form  only  a  single  image.  If,  however, 
we  look  obliquely  at  the  image  of  a 
candle  in  a  looking-glass,  we  shall  see 
several  images,  the  first  feeble,  the  next 
bright,  and  the  others  diminishing  in 
intensity.  The  ray  from  A  is  in  part 
reflected  to  the  eye  from  the  glass  at  b, 

and  gives  rise  to  the  image  a ;  the  re- 
mainder passes  on  and  is  reflected  from  the  metallic  surface 

*  The  perpendiculars  are  not  given  in  the  figures  of  the  book,  as  the  pupil  at  red 
tation  should  draw  all  the  cuts  on  the  blackboard,  erect  the  perpendiculars  and  demon- 
strate the  locati&n  of  the  reflected  ray.  It  wiil  aid  in  drawing  the  perpendicular  to  a 


FIG.  136. 


REFLECTION   OF   LIGHT.  153 

at  c,  and  coming  to  the  eye  forms  a  second  image  a'.  The 
ray  cd,  when  leaving  the  glass  at  d,  loses  a  part,  which  is 
reflected  back  to  form  a  third  image.  This  ray  in  turn  is 
divided  to  form  a  fourth,  and  so  on.* 

Images  seen  in  water  are  symmetrical,  but  inverted.  The 
reason  of  this  can  be  understood  by  holding  an  object  in 
front  of  a  horizontal  looking-glass  and  noticing  the  angle  at 
which  the  rays  must  strike  the  surface  in  order  to  be  re- 
flected to  the  eye.  When  the  moon  is  high  in  the  heavens, 
we  see  the  image  in  the  water  at  only  one  spot,  while  the 
rest  of  the  surface  appears  dark.  The  light  falls  upon  all 
parts,  but  the  rays  are  reflected  from  only  one  point  at  the 

convex  or  concave  surface,  to  remember  that  it  is  a  radius  of  the  sphere  of  which  the 
mirror  forms  a  part.  A  book  held  in  various  positions  before  a  looking-glass  illus- 
trates the  action  of  plane  mirrors.  A  beam  of  light  admitted  into  a  dark  room  and 
reflected  from  a  mirror  will  show  that  the  angles  of  incidence  and  reflection  are  in 
the  same  plane.  Many  of  the  grotesque  effects  of  concave  and  convex  mirrors  may 
be  seen  on  the  inner  and  outer  surfaces  of  a  bright  spoon,  call-bell,  or  metal  cup  (see 
Mayer  &  BarnarcTs  Light  for  inexpensive  experiments). 

*  To  illustrate  the  formation  of  multiple  images,  place  two  small  mirrors  as  in 
Fig.  139,  where  two  coincident  images  are  produced  by  second  partial  reflections.  To 
vary  the  experiment 

hold  the  mirrors  to-  FIG.  139. 

getherlike  the  covers 
of  a  book  placed  on 
end,  and  put  the  can- 
dle between  them  on 
the  table,openingand 
shutting  the  mirror- 
cover  so  as  to  vary  the 
angle ;  or  hold  the 
mirrors  parallel  to 
each  other  with  the 
light  between  them. 
When  the  mirrors  are 
inclined  at  90°,  three 
images  are  -formed ; 
at  60%  five  images  ; 
and  at  45°,  seven  im- 
ages. As  the  angle 
increases,  thenumbcr 
diminishes.  The  im- 
ages are  upon  the  cir- 
cumference of  a  circle 

whose  centre  is  on  a  line  in  which  the  reflecting  surfaces  would  Intersect  if  produced. 
Where  the  mirrors  are  parallel  the  images  are  in  a  straight  line.  They  become  dim- 
mer as  they  recede,  light  being  lost  at  each  reflection.— The  Kaleidoscope  contains 
three  mirrors  set  at  an  angle  of  60°.  Small  bits  of  colored  glass  at  one  end  reflect  to 
the  eye  at  the  other  multiple  images,  which  change  in  varying  patterns  as  the  tube  is 
revolved. 


154 


OPTICS. 


right  angle  to  reach  the  eye.     Each  observer  sees  the  image 
at  a  different  place.     When  the  surface  of  the  water  is  ruf- 


Fio.  140. 


FIG.  141. 


fled,  a  tremulous  line  of  light  is  reflected  from  the  side  of 
each  tiny  wave  that  is  turned  towards  us.  As  every  little 
billow  rises,  it  flashes  a  gleam  of  light  to  our  eyes,  and  then 
sinking,  comes  up  beyond,  to  reflect  another  ray. 

(2.)  A  CON- 
CAVE MlKEOR 
tends  to  collect  the 
rays  of  light.  * 
Thus  in  Fig.  141, 
parallel  rays  f  all- 
ing  upon  the  mir- 

*  This  statement  is  convenient  as  it  is  true  in  the  practical  use  of  the  mirror,  but 
does  not  obtain  in  every  possible  position.  Thus,  if  a  light  be  placed  between  A  and 
F  the  rays  would  be  scattered,  as  can  easily  be  shown  by  a  diagram.  Again,  in 
elementary  optics  it  is  supposed  that  MCN,  known  as  the  angular  aperture  of  the 
mirror,  does  not  exceed  8°  or  10°.  When  greater,  the  rays  reflected  near  the  edge  of 
the  mirror  meet  the  principal  axis  AL,  nearer  the  mirror  than  F.  This  is  called  the 
aberration  of  the  mirror  (p.  161).  The  reflected  rays  will  then  cross  at  points  in  a 
curved  surface  called  a  caustic.  A  section  of  such  a  curve  can  be  seen  when  the  light 
of  a  candle  is  reflected  from  the  inside  of  a  cup  partly  full  of  milk. 


REFLECTION    OF    LIGHT. 


155 


ror  MN  are  reflected  to  the  point  F,  the  principal  focus 
(focus,  a  hearth).  This  is  half  way  between  the  mirror  and 
C,  the  centre  of  curvature,  i.  e.  the  centre  of  the  hollow 
sphere  of  which  the  mirror  is  a  part.  AF  is  the  focal  dis- 
tance ;  CB,  CD,  etc.,  are  radii  of  the  sphere  (perpendiculars, 
to  find  the  angle  of  incidence)  ;  and  the  angles  HBO,  GDC, 
etc.,  are  equal  respectively  to  FBC,  FDC,  etc.  A  light  held 
at  C  will  have  its  rays  brought  to  a  focus  at  C,  where  a  real 
image  will  be  formed  ;  while  one  at  F  will  be  reflected  in  a 
beam  of  parallel  rays. 

Images  formed  ly  concave  mirrors.  Hang  a  concave  mir- 
ror against  the  wall,  and  stand  closely  to  it  between  the 
mirror  and  the  principal  focus.  The  image  is  erect,  virtual, 


FiO.  142. 


and  larger  than  life.  The  ray  a  falls  upon 
the  mirror,  is  reflected  and  strikes  the  eye  as 
if  it  came  from  A.  In  the  same  manner  I  is  seen  at  B. 
The  visual  angle  is  increased  the  nearer  we  approach  the 
mirror,  and  hence  the  larger  the  image  appears.  We 
now  walk  back.  When  we  reach  the  focus,  the  image  dis- 
appears. We  are  in  the  position  of  the  candle  ab  (Fig.  143) 
and  the  real  image  is  behind  us  at  AB.  A  few  of  the  parallel 
rays,  however,  enter  the  eye,  and  an  indistinct  image  is 
formed.  Retiring  still  further,  we  come  to  the  centre  of 
curvature,  Here  we  find  no  distinct  image,  although  por  • 


156 


OPTICS. 


tions  of  our  figure,  as  we  catch  snatches  of  the  rays  forming 
the  image  AB,  are  seen  grotesquely  magnified.  As  we  con- 
tinue to  recede,  we  reach  a  point  beyond  the  centre  of 


FIG.  143. 


curvature.  Here  we  occupy  the  position  AB  (Fig.  143),  and 
see  the  image  at  rib  inverted,  as  the  rays  have  crossed. 
The  points  occupied  by  the  two  candles,  db  and  AB,  are 
termed  conjugate  foci,  because  a  light  at  either  one  is  brought 
to  a  focus  at  the  other. 

FIG.  145. 
FIG.  144. 


(3.)  A  CONVEX  MIBROE  tends  to 
scatter  the  rays  of  light.  The  par-' 
illel  rays  AD  and  BK  (Fig.  144), 
are  reflected  in  the  diverging  lines 
DE  and  KH.  An  eye  receiving 
these  rays  will  perceive  the  image  of 
AB  at  ab,  virtual,  erect,  and  smaller 
than  life.  Whatever  may  be  the 
position  of  the  object,  the  image 
being  always  between  the  object  and 
the  centre  of  curvature  is  smaller 
than  the  object. 

4.  Total  Reflection. — When  we  look  obliquely  into  n 


REFRACTION    OF    LIGHT. 


157 


pond,  we  cannot  see  the  bottom,  because  the  rays  of  light 
from  belo\v  are  reflected  downward  at  the  surface  of  the 
water.  Hold  a  glass  of  water  above  the  level  of  the  eye,  and 
the  upper  part  will  gleam  like  burnished  silver.  *  Thus  the 
internal  surface  of  a  transparent  body  becomes  a  mirror. 
This  occurs  when  light  would  pass  very  obliquely  from  a 
denser  to  a  rarer  medium. 


3.— REFRACTION  OF  LIGHT. 

1.   Definition. — When  a  ray   of  light  passes  obliquely  | 
from  one  medium   to  another  of   different  density,  it  is  I 
refracted  or  bent  out  of  its  course.     Ex. :  A  spoon  in  clear 
tea  appears  bent. — An  oar  dipping  in  still  water  seems  to 
break  at  the  point  where  it  enters  the 'water. f — Put  a  cent 
in  a  bowl.     Standing  where  you  cannot  see  the  coin,  let 
another  person  pour  water  into  the  vessel,  when   the  coin 

*  Place  a  "bright  spoon  in  the  glass  and  notice  its  image  reflected  from  the  surface 
of  the  water.  The  apparently  increased  size  of  the  spoon,  the  broken  handle,  etc., 
will  be  understood  after  reading  the  next  subject.  Turn  the  spoon  about  in  the  glass 
and,  changing  the  angle  of  observation,  notice  the  effect.  The  real  handle  may  ap- 
parently be  attached  to  the  image  in  the  water.  The  spoon  will  soon  be  covered 
with  bubbles  of  air  shining,  like  pearls,  from  total  reflection.  This  shows  also  the 
presence  of  air  in  water  and 

the  adhesion  of  gases  to  solids.  FIG.  146. 

The  goblet,  if  filled  with  cold 
water,  will  "sweat,"  as  it  is 
called,  from  the  condensed 
moisture  of  the  atmosphere. 

t  Fish  seem  nearer  the  sur- 
face than  they  really  are,  and 
Indians,  who  spear  them,  try  to 
strike  perpendicularly,  or  else 
aim  lower  than  they  apparently 
lie.— In  Fig.  146,  the  man  on  the 
bridge  sees  the  fish  in  its  true 
place  :  but  the  boy  on  the  bank 
sees  the  'fish  at  a,  while  the  fish 
sees  the  boy  at  c.— Water  is 
deeper  than  it  appears.  Look 
obliquely  into  a  pail  of  water, 
then  place  your  finger  on  the 
outs' de  where  the  bottom  seems 
to  be ;  you  will  be  surprised  to 

find  the  real  bottom  is  several  inches  below.— Fill  a  glass  dish  with  water,  and, 
darkening  the  windows,  let  a  sunbeam  fall  upon  the  surface.  The  ray  will  bend  as 
it  enters.  Dust  scattered  through  the  air  will  make  the  beam  distinct. 


158 


OPTICS. 


•will  be  lifted  into   view. 


Fie.  147. 


To  understand   the  apparent 
^      change  of  position,    re- 
member that  the  object  is 
seen   in  the  direction  of 
the  refracted  ray  as  it 
enters  the  eye.     Let  L, 
Fig.  148,  be  a  body  be- 
neath the  water.    A  ray,  LA,  coming 
to  the  surface,  is  bent 

downward  toward   C,          FIO.  148. 

and  strikes  the  eye  as 
if  it  came  from  L'.  The  object  will  there- 
fore apparently  be  elevated  above  its  true 
place. 

2.  Laws  of  Refraction.— I.  In  passing 
into  a  rarer  medium,  the  ray  is  bent  from 
the  perpendicular.  II.  In  passing  into  a 
denser  medium,  the  ray  is  bent  toward  the  perpendicular.* 

ILLUSTRATIONS. — Path  of  rays  through  a  window-glass. — 
When  a  ray  enters  a  window-glass, 
it  is  refracted  toward  the  perpen- 
dicular (2d  law),  and  on  leaving,  is 
refracted  from  the  perpendicular 
(1st  law).  The  general  direction 
of  objects  is  therefore  unchanged. 
A  poor  quality  of  glass  produces 
distortion  by  its  unequal  density 
and  uneven  surface. 

/IU  Path  of  rays  through  a  prism. 

A  ray  of  light,  on  entering  and  on 
leaving  a  prism,  is  refracted  as  by 
a  window-glass.     The  inclination  of  the  sides  causes  the  ray 

*  Both  the  Incident  and  the  refracted  ray  lie  in  the  same  plane  as  the  normal 
(perpendicular).  The  ratio  between  the  sines  of  the  incident  and  refracted  angles  is 
termed  the  index  of  refraction.  It  varies  with  the  media.  Ex. :  From  air  to  water  ii 
la  land  from  air  to  glass  | . 


FIG.  149. 


ff 


BKFRACTIOH    OF    LlGflT. 


to  be  bent  twice  in 
the  same  direction. 
The  candle  L  will 
therefore  appear  to 
be  at  r. 

3.  Lenses, — A 

lens  is  a  transparent 
body,  with  at  least 
one  curved  surface. 
There  are  two  gen- 
eral classes  of  lenses,  concave  and  convex.*  (See  Fig.  151.) 

FIG.  151. 


(1.)  THE  DOUBLE-COKVEX  LENS  has  two  convex  surfaces. 
Its  action  on  light  is  like  that  of  a  concave  mirror.    A  ray 


FIG.  152. 


X  striking  perpendicularly,  is  not  refracted.  The  parallel 
rays  M,  L,  etc.,  are  refracted  both  on  entering  and  on  leav- 
ing the  lens,  and  are  converged  at  F,  the  focus,  f  If  a  light 
be  placed  at  F,  its  rays  will  be  made  parallel. 

*  Forms  of  lenses:  M,  double-convex;  N,  plano-convex ;  O,  meniscus  (crescent); 
P,  double-concave  ;  Q,  plano-concave ;  R,  concavo-convex.  The  first  three  are 
styled  magnifiers,  and  the  second,  diminishers. 

t  The  convex  lens  is  sometimes  termed  a  burning-glass,  being  used,  like  the 


160  OPTICS. 

The  image  formed  by  a  convex  lens  is  like  that  of  a  con- 
cave mirror.  If  we  hold  a  lens  above  a  printed  page,  when 
we  obtain  the  focal  distance  correctly,  we  shall  find  the  let- 


Fio.  153. 


ters  right-side  up  and  highly  magnified.  In  Fig.  153  we 
see  how  the  converging  power  of  the  lens  increases  the 
visual  angle,  and  makes  the  object  AB  appear  the  size  ah. 


FIG.  154. 


Moving  the  lens  back  from  the  page,  the  letters  entirely  dis- 
appear as  we  pass  the  principal  focus.  At  length  they 
reappear  again,  but  smaller  and  inverted  (Fig.  154). 

(2.)  THE  DOUBLE-CONCAYE  LENS  has  two  concave  sur- 
faces. Its  action  on  light  is  like  that  of  a  convex  mirror. 
Thus,  diverging  rays  from  L  (Fig.  155)  are  rendered  more 
diverging,  and,  to  an  eye  which  receives  the  rays  MN",  the 
candle  would  seem  to  be  at  Z,  where  the  image  is  seen. 

concave  mirror,  for  collecting  the  sun's  rays.  Lenses  have  been  manufactured  of 
sufficient  power  to  melt  a  stone  by  sunheat.  Even  glass-globes  of  water,  puch  as 
are  used  for  gold  fishes  or  in  the  windows  of  drug  stores,  may  fire  adjacent  objects. 


REFRACTIOH    OF    LIGHT. 
FIG.  155. 


163 


The  image  formed  by  a  concave  lens,  like  that  of  a  convex 
mirror,  is  virtual,  erect,  and  diminished  in  size  (Fig.  156). 


FIG.  156. 


4.  Aberration. — Bays  which  pass  through  a  lens  near 
the  edge  are  brought  to  a  focus  sooner  than  those  near  the 
centre.  Therefore,  when  the  border  of  an  image  is  clear, 
the  centre  will  be  indistinct,  and  vice  versa.  This  wander- 
ing of  the  rays  from  the  focus  is  termed  spherical  aberration. 
The  different  refrangibility  of  the  colors  which  compose 
white  light  (p.  163)  produces  chromatic  aberration.  The 
violet,  being  bent  most,  comes  to  a  focus  sooner  than  the  red, 
which  is  bent  least.  This  causes  the  play  of  colors  seen 
around  the  image  produced  bv  an  ordinary  lens.  The  defect 


162 


OPTICS. 


is  remedied  by  a  second  lens  of  different  dispersive  power, 
which  counteracts  the  effect  of  the  first.  Such  a  compound 
lens  is  said  to  be  achromatic  (colorless). 

5.  Mirage. — In  the  heated  deserts  of  Africa,  the  trav- 
eller sometimes  sees  in  the  distance  quiet  lakes  with  the 
shadows  of  trees  in  their  cool  waters.     Rushing  forward  to 
slake  his  eager  thirst,  he  finds  only  the  barren  waste  of  sand. 
The  mariner  often  recognizes  in  the  sky  the  images  of  ships, 
and  the  far-distant  coast,  with  its  familiar  cliffs.     The  cause 
/      of  these  phenomena  is  the  refraction  and  reflection  of  the 
(       rays  of  light  traversing  layers  of   air  of   unequal  density. 

FIG.  157. 


Sometimes  a  layer  of  air  high  up  in  the  sky  acts  as  a 
reflector,  and  sends  down  inverted  images  of  ships  which 
are  beyond  the  horizon.  In  Fig.  157,  rays  of  light  from  a 
clump  of  trees  are  refracted  more  and  more  until  finally  they 
are  reflected  from  a  layer  at  a,  and  enter  the  eye  of  the  Arab 
as  if  they  came  from  the  surface  of  a  quiet  lake.  The 
deception  is  made  complete  by  the  fact  that  the  sandy  desert, 
shimmering  in  the  hot  sun,  often  has  in  the  distance  the 
aspect  of  tranquil  water.* 

*  Hold  a  pane  of  glass  horizontally  above  the  eyes.    The  inverted  images  of  objects 
in  front  may  be  seen,  reflected  from  the  surface  of  the  glass. 


COMPOSITION    OF    LIGHT. 


163 


4.    THE   COMPOSITION   OF    LIGHT. 

1.  Solar  Spectrum. — When  a  sunbeam  shines  through 
a  prism,  the  ray  is  not  only  bent  from  its  course,  but  is  also 
spread  out,  fan-like,  into  a  band  of  rainbow-colors — the 
solar  spectrum.  It  contains  the  seven  primary  colors — 
violet,  indigo,  blue,  green,  yellow,  orange,  red.*  If  we  receive 
the  spectrum  on  a  concave  mirror,  or  pass  it  through  a 
convex  lens,  it  will  form  a  white  spot.  We  therefore  con- 
clude that  white  light  is  composed  of  seven  colors.  They 
are  separated  because  the  prism  bends  them  unequally. 
The  violet  is  most  refracted,  and  the  red  least. 


FIG.  158. 


2.  Three  classes  of  rays  exist  in  the  solar  spectrum  •, 
viz. :  the  heat  rays ;  luminous  rays  ;  and  actinic 
rays.f  If~we  examine  the  prismatic  spectrum  with  a  very 
delicate  thermometer,  we  find  that  the  heat  increases  from 
the  violet  to  the  red  end,  and  becomes  the  greatest  in  the 
dark  space  just  beyond.  If  we  test  with  a  paper  containing 

*  Notice  that  the  initial  letters  spell  the  mnemonic  word,  Vib-gy-or, 
\  The  classification  into  three  kinds  of  rays  is  retained  as  it  is  etill  common  in 
scientific  books.  Draper  has  shown  that  the  effects  described  above  are  due  merely 
to  an  unequal  distribution  of  the  ether- waves  by  the  prism.  Rays  of  all  colors  have 
the  same  light,  heat,  and  chemical  power,  and  the  same  cause— radiant  energy.  We 
call  this  one  thing,  light,  heat,  or  actinism,  according  to  the  means  used  to  reveal  its 
presence  (pp.  183-4).  See  also  p.  xi.  Fresh  Facts,  etc. 


chloride  of  silver,  it  will  blacken  least  in  the  red,  most 
toward  the  violet,  and  some  in  the  dark  space  beyond.  Be- 
tween these  two  extremes  lie  the  rays  which  strongly  affect 
the  eye.  All  are  mingled  in  the  normal  spectrum. 

3.  Three  Kinds  of  Spectra.— I.  When  the  light  of  a 
solid  or  liquid  body,  as  iron  white-hot,  is  passed  through  a 
prism,  the  spectrum  is  continuous  and  consists  of  the  familiar 
colors  of  the  rainbow.  II.  When  the  light  of  a  burning 
gas  is  passed  through  a  prism,  the  spectrum  is  not  con- 
tinuous,  but  consists  of  bright-colored  lines — copper  giving 
a  set  of  green  lines,  and  zinc  one  of  bright  -blue  and  red. 

Each  element  produces  a 
series  which  can  be  recog- 
nized as  its  test.  III. 
When  a  light  of  the  first 
kind  is  passed  through 
one  of  the  second  at  a 
lower  temperature,  the 
spectrum  is  crossed  ly 
dark  lines.  Thus,  when 
the  light  of  white  hot 

lime  shines  through  a  flame  of  burning  sodium,  instead 
of  the  two  Tivid  yellow  lines  so  characteristic  of  that  metal, 
two  black  lines  occupy  their  place.  In  general,  a  gaseous 
flame  absorbs  rays  of  the  same  color  that  it  emits.* 


FIG.  160. 


*  Imagine  a  room  filled  with  piano- wires,  stretched  in  every  direction  and  tuned 
to  one  key.    Now  let  a  person  at  one  end  of  the  room  play  a  tune.    Another  per- 


OF  LIGHT.  165 

4.  The  Spectroscope  is  an  instrument  for  examining 
spectra.  The  rays  of  light  (Fig.  161)  enter  through  a  nar- 
row slit  in  the  tube  at  A,  and  are  rendered  parallel  by  an 
object-glass.  They  then  pass  through  the  prisms  at  0,  are 


separated  into  the  different  colors,  and  entering  the  tele- 
scope at  D,  fall  upon  the  eye  at  B.  Any  substance  may 
be  placed  in  the  flame  in  front  of  A  and  its  spectrum 
examined.* 

5.  The  Rainbow  is  formed  by  the  refraction  and  re-- 
flection of  the  sunbeam  in  drops  of  falling  water.  The 
wHIte  light  is  thus  decomposed  into  its  simple  colors.  The 

son  at  the  opposite  end  of  the  room  would  hear  the  tune  perfectly  except  when 
the  particular  note  which  belonged  to  the  wires  was  struck,  when  that  would  be 
sifted  out. 

*  On  the  uses  of  the  spectroscope,  examine  Astroncmy,  p.  286,  and  Chemistry, 
p.  145.  The  frontispiece  of  the  latter  gives  a  colored  illustration  of  the  spectra.— 
The  solar  spectrum  is  crossed  by  dark  lines  known  as  FraunJiofer's  lines.  The  most 
prominent  are  marked  for  convenience  of  reference  (A,  B,  C,  etc.,  Fig.  160).  The 
spectroscope  affords  an  unrivalled  mode  of  analysis.  No  cLemical  test  is  so  delicate. 
Strike  together  two  books  near  the  light  at  the  slit  of  the  spectroscope,  and  the  dust 
blown  into  the  flame  will  contain  enough  sodium  (the  basis  of  common  salt)  to  cause 
the  yellow  D  lines— its  test— to  flash  out  distinctly.  (Seo  note  on  spectroscope, 
p.  226.)  A  very  effective  spectroscope  may  be  contrived  thus:  Cut  a  slit  not  over 
3>o  inch  wide  and  2  inches  long  in  a  piece  of  tinfoil,  and  guia  it  on  a  pane  of  glass. 
Hold  this  before  a  flame  and  look  at  it  through  a  prism. 


166 


OPTICS. 


inner  arch  is  termed  the  primary  bow  ;  the  outer  or  fainter 
arch,  the  secondary. 

PEIMAKY  Bow. — A  ray  of  light,  S",  enters,  and  is 
bent  downward  at  the  top  of  a  falling  drop,  passes  to  the 
opposite  side,  is  there  reflected,  then  passing  out  of  the 
lower  side,  is  bent  upward.  By  the  refraction  the  ray  of 
white  light  is  decomposed,  so  that  when  it  emerges  it  is 
spread  out  fan-like,  as  in  the  solar  spectrum.  Suppose  that 
the  eye  of  a  spectator  is  in  a  proper  position  to  receive  the 
red  ray,  he  cannot  receive  any  other  color  from  the  same 
drop,  because  the  red  is  bent  upward  the  least,  and  all  the 
others  will  pass  directly  over  his  head.  He  sees  the  violet 
in  a  drop  below.  Intermediate  drops  furnish  the  other 
colors  of  the  spectrum. 

FIG.  163. 


SECONDARY  Bow. — A  ray  of  light,  S,  strikes  the  bot- 
tom of  a  drop,  v,  is  refracted  upward,  passes  to  the  oppo- 
site side,  where  it  is  twice  reflected,  and  thence  passes  out 
at  the  upper  side  of  the  drop.  The  violet  ray  being  most 
refracted,  is  bent  down  to  the  eye  of  the  spectator.  Another 
drop,  r,  refracting  another  ray  of  light,  is  in  the  right  posi- 
tion to  send  the  red  ray  to  the  eye. 


COMPOSITION    OF    LIGHT. 


167 


WHY  THE  Bow  is  OIECULAE. — When  the  red  ray  of 
the  primary  bow  leaves  the  drop,  it  forms  an  angle  with  the 
sun's  ray,  S'V,  of  about  42°,  and  the  violet  40°.  These  angles 
are  constant.  Let  ab  be  a  straight  line  drawn  from  the  sun 
through  the  observer's  eye.  If  produced,  it  would  pass 
through  the  centre  of  the  circle  of  which  the  rainbow  is  an 
arc.  This  line  is  termed  the  visual  axis.  It  is  parallel  to 
the  rays  of  the  sun  ;  and  when  it  is  also  parallel  to  the 
horizon,  the  rainbow  is  a  semicircle.  Suppose  the  line  Ey 
in  the  primary  bow  to  be  revolved  around  E#,  keeping 
the  angle  #Ev  unchanged ;  the  point  v  would  describe  a 
circle  on  the  sky,  and  every  drop  over  which  it  passed  would 
be  at  the  proper  angle  to  send  a  violet  ray  to  the  eye  at  E. 
Imagine  the  same  with  the  drop  r.  We  can  thus  see  (a)  the 
bow  must  be  circular ;  (b)  when  the  sun  is  high  in  the 
heavens,  the  whole  bow  sinks  below  the  horizon ;  (c)  the 
lower  the  sun  the  larger  is 
the  visible  circumference ; 
and  (d)  on  lofty  mountains 
a  perfect  circle  may  some- 
times be  seen.* 


GREENISH 


10WISH  OREEN 


BLUE 


YELLOW 


ORANGE 


H  ORANGE 


6.  Complementary 
Colors.  — Two  colors, 
which  by  their  mixture 
produce  white  light,  are 
termed  complementary  to 
each  other.  Thus,  if  we 
sift  the  red  rays  out  of  a 
beam  of  light  and  bring 
the  remainder  to  a  focus, 

a  green  image  will  be  formed,  f     In  Fig.  163  the  colors 
opposite  each  other  are  complementary.     Place  a  red  and  a 

*  Halos,  coronas,  sundogs,  circles  about  the  moon,  and  the  tinting  at  sunrise  and 
sunset,  are  produced  by  the  refraction  and  reflection  of  the  sun's  rays  by  the  clouds. 
The  phenomenon  known  as  the  "  sun's  drawing  water,"  consists  of  the  long  shadows 
of  broken  clouds.  Twilight  and  kindred  topics  are  treated  in  Astronomy. 

t  Certain  substances  are  able  to  split  a  ray  of  light  into  its  complementary  colors. 
Thus  gold-leaf  reflects  the  red  and  transmits  the  green. 


168  OPTICS. 

blue  ribbon  side  by  side.  The  former  will  take  on  a  yellow 
and  the  latter  a  green  tint.  Lay  a  piece  of  tissue  paper 
upon  black  letters  printed  on  colored  paper.  The  dark 
letters  will  appear  of  a  color  complementary  to  that  of  the 
background.* 

7.  Interference  of  Light  (Netvton's  Rings). — Let  the 
convex  side  of  a  plano-convex  lens  be  pressed  down  upon  a 
plane  of  glass.     The  two  surfaces  will  apparently  touch  at 
the   centre.      If  different  circles  be 
described  around  this  point,  at  all   ^^^     FlG' 164'    ^^ 

"OtlluS    01    GtLC-Ll   Gi.l*C-L6   "LUG  SU-J/IclC'tJo  AVJ.-L1     \y///////////////.' ,  /.  '  ''/////////////////////////ih 

be  the  same  distance  apart,  and  the 

larger  the  circle  the  greater  the  distance.  Now  let  a  beam 
of  red  light  fall  upon  the  flat  surface.  A  black  spot  is  seen 
at  the  centre  ;  around  this  a  circle  of  red  light,  then  a  dark 
ring,  then  another  circle  of  red  light,  and  so  alternating  to 
the  circumference.  The  distances  between  the  surfaces  of 
the  glass,  where  the  successive  dark  rings  appear,  are  pro- 
portional to  the  numbers  0,  2,  4 ,  and  the  bright 

circles  to  1,  3,  5 This  fact  suggests  the  cause.     There 

are  two  sets  of  waves,  one  reflected  from  the  upper  surface 
of  the  plane  glass,  and  the  other  from  the  lower  surface  of 
the  convex  glass.  These  alternately  interfere,  producing 
darkness,  and  combine,  making  an  intenser  color,  f  To  de- 

*  A  color  is  heightened  when  placed  near  its  complement.  A  red  apple  is  the 
brighter  for  the  contrast  of  the  green  leaf.— Observe  a  white  cloud  through  a  bit  of 
red  glass  with  one  eye  and  through  green  glass  with  the  other  eye.  After  some 
moments,  transfer  both  eyes  to  the  red  glass,  opening  and  closing  them  alternately. 
The  strengthening  of  the  red  color  in  the  eye  fatigued  by  its  complementary  green, 
is  very  striking.— In  examining  ribbons  of  the  same  color,  the  eye  becomes  v/earied 
and  unable  to  detect  the  shade,  because  of  the  mingling  of  the  complementary  hue. 

t  The  play  of  colors  in  mother-of-pearl  is  due  to  the  interference  of  light  in  its 
thin  overlapping  plates.— In  a  similar  manner  the  plumage  of  certain  birds  reflects 
changeable  hues. — A  metallic  surface  ruled  with  fine  parallel  lines  not  more  than 
55Vo  of  an  inch  apart,  gleams  with  brilliant  colors.— Thin  cracks  in  plates  of  glass  or 
quartz,  mica  when  two  layers  are  slightly  separated,  even  the  scum  floating  in 
stagnant  water,  breaks  up  the  white  light  of  the  sunbeam  and  reflects  the  varying 
tints  of  the  rainbow.— The  rich  coloring  of  a  soap-bubble  is  caused  by  the  interfer- 
ence of  the  rays  reflected  from  the  upper  and  lower  surfaces  of  the  bubble.— 
DIFFRACTION  is  a  kind  of  interference  produced  by  a  beam  of  light  passing  along  the 
edge  of  an  opaque  body  or  through  a  small  opening.  Ex.  :  If  we  hold  a  fine  needle 
close  to  one  eye  and  look  toward  the  window,  we  see  several  needles.— Place  the 


COMPOSITION    OF    LIGH-T.  169 

termine  the  length  of  a  wave  of  red  light,  we  have  only  to 
measure  the  distance  between  the  two  glasses  at  the  first 
ring. 

When  beams  of  light  of  the  various  colors  are  used  cor- 
responding circles  are  obtained,  having  different  diameters ; 
red  light  gives  the  !argest,  and  violet  the  smallest.  We 
hence  conclude  that  red  waves  are  the  longest,  and  violet 
the  shortest.  The  minuteness  of  these  waves  passes  com- 
prehension. About  40,000  red  waves  and  60,000  violet  ones 
are  comprised  within  a  single  inch.  Knowing  the  velocity 
of  light,  we  can  calculate  how  many  of  these  tiny  waves 
reach  our  eyes  each  second.  When  we  look  at  a  violet 
object,  757  million  million  of  ether-waves  break  on  the 
retina  every  moment ! 

8.  Color  is  analogous  to  pitch,  violet  corresponding  to 
the  high  and  red  to  the  low  sounds  in  music.  Intensity  of 
color,  as  of  sound,  depends  on  the  amplitude  of  the  vibra- 
tions. When  a  body  absorbs  all  the  colors  of  the  spectrum 
except  blue,  but  reflects  that  to  the  eye,  we  call  it  a  blue 
body;  when  it  absorbs  all  but  green,  we  call  it  a  green 
body.*  Red  glass  has  the  power  of  absorbing  all  except  the 
red  rays,  which  it  transmits.  When  a  substance  reflects  all 
the  colors  to  the  eye,  it  seems  to  us  white.  If  it  absorbs  all 
the  colors,  it  is  black.  Thus  color  is  not  an  inherent  prop- 
erty of  objects,  f  In  darkness  all  things  are  colorless. 

blades  of  two  knives  closely  together  and  hold  them  up  to  the  sky:  waving  lines  of 
interference  will  shade  the  open  space.— Look  at  the  sky  through  the  meshes  of  a 
veil,  or  at  a  lamp-light  through  a  bird-feather  or  a  fine  slit  in  a  card,  and  delicate 
colors  will  appear. 

*  Some  eyes  are  blind  to  certain  colors,  as  some  ears  are  deaf  to  certain  sounds. 
"  Color-blindness  "  generally  exists  as  to  red.  Such  a  person  cannot  by  the  color 
distinguish  ripe  cherries  from  the  leaves.  Doubtless  railway  accidents  have  occurred 
through  this  inability  to  apprehend  signals.  Dr.  Mitchell  mentions  a  naval  officer 
who  chose  a  blue  coat  and  red  waistcoat,  believing  them  of  the  same  color ;  a  tailor 
who  mended  a  black  silk  waistcoat  with  a  piece  of  crimson  ;  and  another  who  put 
a  red  collar  on  a  blue  coat.  Dalton  could  see  in  the  solar  spectrum  only  two  colors, 
blue  and  yellow,  and  having  once  dropped  a  piece  of  red  sealing-wax  in  the  grass,  he 
could  not  distinguish  it. 

t  Moisten  a  swab  with  alcohol  saturated  with  common  salt.    On  igniting  the 


170 


OPTICS. 


FIG.  165. 


9.  Polarization  of  Light   (Double  Refraction).— If  we 

could  loolf~at  ihe  end  of  a  ray  of  light  as  we  can  at  the  end 
of  a  rod,  we  should  see  the  particles  of  ether  swimming 
swiftly  to  and  fro  in  the  direction  of  all  the 
diameters  (Fig.  165).  Certain  crystals  have  the 
power  of  sifting  and  arranging  these  vibra- 
tions into  two  sets  at  right  angles  to  each 
other,  making  ;t  ray  of  the  form  seen  in 
Fig.  166.  As  one  set  is  more  refracted  than 
the  other,  the  ray  is  divided  into  two — the 
ordinary  and  the  extraordinary.  Rays  which  have  thus  been 
sifted  constitute  polarized  light.  Iceland  spar  possesses  the 


FIG.  166. 


FIG.  167. 


property  of  double  refraction  in  a  remarkable  degree.  An 
object  viewed  through  it  appears  double.  If  the  crystal  be 
placed  on  a  dot  and  slowly 
turned  round,  two  dots 
will  be  seen,  the  second 
revolving  about  the  first. 

Objects  seen  by  polar- 
ized light  present  curious 
changes.  A  crystal  of 
quartz  reveals  beautiful 
colors  due  to  interference. 
Looking  at  a  lamp-light  through  a  piece  of  thin  mica,  we  see 
a  series  of  polarised  rays  having  a  star-like  form.  When 
polarized  light  is  passed  through  common  glass  no  change 
is  noticed,  but  on  slight  pressure  a  system  of  variegated 
colors  appears.  Polarized  light  therefore  affords  a  delicate 
means  of  determining  the  molecular  structure  of  a  body.* 

*  Some  substances  have  the  power  of  twisting  the  plane  of  the  polarized  light. 
Cane-sugar  turns  it  to  the  right,  and  fruit-sugar  to  the  left  (Chemistry,  p.  190). 
The  French  government  uses  a  polarizing  instrument,  in  which  this  principle  10 
applied  to  test  the  quality  of  sugar. 


OPTICAL    INSTRUMENTS. 
5.— OPTICAL  INSTRUMENTS. 


171 


1.  Microscopes  (to  see  small  things)  are  of  two  kinds, 
simple  and  compound.  The  former  consists  of  one  or  more 
convex  lenses  through  which  the  object  is  seen  directly  :  the 
latter  contains  a  simple  magnifier  for  viewing  the  image  of 

Fia  168. 


an  object  produced  by  a  second  lens.  Fig.  168  represents  a 
compound  microscope.  At  M  is  a  mirror  which  reflects  the 
rays  of  light  through  the  object  a.  The  object-lens  (objec- 
tive) o  forms,  in  the  tube  above,  a  magnified,  inverted  image 
of  the  object.  The  eye-lens  0  (ocular)  magnifies  this  image. 


172  OPTICS. 

The  magnifying  power  of  the  instrument  is  nearly  equal  to 
the  product  of  that  of  the  two  lenses.  If  a  microscope  in- 
creases the  apparent  diameter  of  an  object  100  times,  it  is 
said  to  have  a  power  of  100  diameters,  the  surface  being 
magnified  1002  =10,000  times.  The  eye-piece  may  be  only  a 
single  lens,  and  is  really  a  simple  microscope.  The  object- 
lens  often  consists  of  several  lenses,  and  each  one  of  a  com- 
bination (p.  161)  to  prevent  aberration. 

2.  Telescopes  (to  see  afar  off)  are  of  two  kinds,  reflect- 
ing and  refracting.  The  former  contains  a  large  metallic 
mirror  (speculum)  which  reflects  the  rays  of  light  to  a  focus. 
The  observer  stands  at  the  side  and  examines  the  image  with 
an  eye-piece.* 

The  Refracting  Telescope  contains  an  object-lens  o  which 
forms  an  image  ab.  This  is  viewed  through  the  eye-piece  0, 
which  produces  a  magnified,  inverted  image  cd.  The  latter 
unage  is  as  much  larger  than  the  former  as  the  focal  distance 

FIG.  169. 


of  the  eye-piece  is  less  than  that  of  the  object-glass.  The 
larger  the  object-lens  the  more  light  is  collected  with  which 
to  view  the  image.  The  magnifying  power  is  principally  due 
to  the  eye-piece,  f  The  apparent  inversion  of  the  object  is 

*  The  largest  reflecting  telescope  is  that  of  Lord  Kosse  (See  Frontispiece  to 
Astronomy).  Its  speculum  is  6  feet  in  diameter  and  gathers  about  120,000  times  as 
much  light  as  would  ordinarily  enter  the  eye. 

t  The  Washington  Observatory  telescope  has  an  object-glass  26  inches  in  diam- 
eter, and  of  excellent  defining  power.  The  Chicago  telescope  has  a  lens  of  18£  inches 
diameter.  It  collects  *•  5000  times  as  much  light  as  the  unaided  pupil  "—equivalent 
to  increasing  the  astronomer's  eye  to  that  size.  The  use  of  the  telescope  depends 
upon  (1st)  its  light-collecting  and  (2d)  its  magnifying  power.  Thus  Herschel,  illus- 
trating the  former  point,  says  that  once  he  told  the  time  of  night  from  a  clock  on  a 
steeple  invisible  on  account  of  the  darkness.  It  is  noticeable  that  while  in  the  com- 
pound microscope  the  image  is  as  much  larger  than  the  object  as  the  image  is  further 


OPTICAL    INSTRUMENTS. 
Pie.  170. 


173 


CAMBRIDGE  EQUATORIAL. 

of  no  importance  for  astronomical  purposes.     In  terrestrial 
observations  additional  lenses  are  used  to  invert  the  image. 

3.  The  Opera-glass  contains  an  object-glass  0  and  an 

FIG.  171. 


than  the  object  from  the  object-glass,  in  the  telescope  the  image  is  as  much  smaller 
than  the  object  as  it  is  nearer  than  the  object  to  the  object-glass ;  while  in  both  cases 
the  image  is  examined  with  a  magnifier.  If  a  power  of  1000  be  used  in  looking  at 
the  sun,  we  shall  evidently  see  the  sun  as  if  it  were  only  93,000  miles  away,  or  less 
than  one-half  the  distance  of  the  moon.  The  same  power  used  upon  the  moo» 
vould  bring  that  body  apparently  to  within  240  miles  of  us. 


174 


OPTICS. 


Tie.  178. 


opticon,  contains 
a  light  (Fig.  174) 
upon  the  object 
magnified  image 
produced  by  two 
to  melt  into  each 


eye-piece  o.  The  latter  is  a  double- 
concave  lens  ;  this  increases  the  visual 
angle  by  diverging  the  rays  of  light, 
which  would  otherwise  come  to  a  focus 
beyond  the  eye-piece.  An  erect  and 
magnified  image  is  seen  at  db. 

4.  The  Stereoscope  contains  por- 
tions of  two  convex  lenses  (Fig.  172). 
Two  photographs  A  and  B  are  taken 
by  two  cameras  inclined  to  each  other. 
This  produces  two  pictures  like  the 
views  we  obtain  of  an  object  by  the 
use   of    each   eye    alternately.      The 
blending  at  C  causes  the  appearance 
of  solidity.* 

5.  The  Magic  Lantern,  or  Stere- 
a  reflector  M,  which  condenses  the  rays  of 
upon  a  lens  L.     They  are  there  converged 

db.  Thence  a  double  lens  m  throws  a 
on  the  screen  AB.  Dissolving  views  are 
lanterns  containing  the  scenes  which  are 
other. 

FIG.  173. 


*  In  Pig.  173  there  are  two  views  of  a  tunnel.  In  one  the  opening  is  at  the  left 
of  the  centre  and  in  the  other  at  the  right.  If  the  view  be  held  about  4  inches  from 
the  eyes  three  engravings  will  be  seen,  the  middle  one  formed  by  the  mental  blend- 
ing of  the  other  two.  By  closing  either  eye  alternately  one  view  will  disappear. 

See  also  p.  xii.    Fresh  Facts  and  Theories 


OPTICAL    INSTRUMENTS. 


175 


FIG.  175. 


6.  The   Camera,   used  by  photographers,  contains  a 
double-convex  lens,  A,  which  throws  an  inverted  image  of 
the    object  upon 

the  ground-glass 
screen  EB.  When 
the  focus  has  been 
obtained,  the 
screen  is  removed 
and  a  slide,  con- 
taining a  sensitive 
film,  is  inserted  in 
its  place.  ( Chem- 
istry, p.  167.) 

7.  The  Eye  is 
a  unique   optical 

instrument  resembling  a  camera.  It  is  rarely,  if  ever, 
troubled  by  spherical  or  chromatic  aberration,  and  is  self- 
focusing.  The  outer  membrane  is  termed  the  sclerotic  coat, 
S.  It  is  tough,  white,  opaque,  and  firm.  A  little  portion 
in  front,  called  the  cornea,  c,  is  more  convex  and  perfectly 
transparent.  The  middle  or  choroid  coat,  C,  is  soft  and 
delicate,  like  velvet.  It  lines  the  inner  part  of  the  eye  and 
is  covered  with  a  black  pigment,  which  absorbs  the  super- 
fluous light.  Over  it  the  optic  nerve,  which  enters  at  the 
rear,  expands  in  a  net-work  of  delicate  fibres  termed  the 


176  OPTICS. 

retina,  the  seat  of  vision.  Back  of  the  cornea  is  a  colored 
curtain,  hi,  the  iris  (rainbow),  in  which  is  a  round  hole 
called  the  pupil.  The  crystalline  lens,  o,  is  a  double- 
convex  lens,  composed  of  concentric  layers  somewhat  like  an 
onion,  weighing  about  4  grains  and  transparent  as  glass. 
Between  the  cornea  and  the  crystalline  lens  is  a  limpid  fluid 

Fie.  176. 


termed  the  aqueous  humor;  while  the  vitreous  humor,  a 
transparent,  jelly-like  liquid,  fills  the  space  back  of  the 
crystalline  lens. 

Let  AB  represent  an  object  in  front  of  the  eye.  Rays  of 
light  are  first  refracted  by  the  aqueous  humor,  next  by  the 
crystalline  lens,  and  last  by  the  vitreous  humor,  forming  on 
the  retina  an  image,  ab,*  which  is  real,  inverted,  and  smaller 
than  the  object.  To  render  vision  distinct,  the  rays  must 
be  accurately  focused  on  the  retina.  If  we  gaze  steadily  at 
an  object  near  by,  and  then  suddenly  observe  a  distant  one, 
we  find  our  vision  blurred.  In  a  few  moments  it  becomes 
clear  again,  showing  that  the  eye  has  the  power  of  adapting 
itself  to  the  varying  distances  of  objects.  This  is  done  by 
a  change  in  the  convexity  of  the  crystalline  lens.  When 

*  The  diameter  of  the  eye  is  less  than  an  inch ;  yet,  as  we  look  over  an  extended 
landscape,  every  feature,  with  all  its  variety  of  shade  and  color,  is  repeated  in 
miniature  on  the  retina.  Millions  upon  millions  of  ether  waves,  converging  from 
every  direction,  break  on  that  tiny  beach,  while  we,  oblivious  to  the  marvellous 
nature  of  the  act,  think  only  of  the  beauty  of  the  revelation.  Yet  in  it  the  physicist 
sees  a  new  illustration  of  the  simplicity  and  perfection  of  the  laws  and  methods  of 
the  Divine  Workman,  and  a  continued  reminder  of  His  forethought  and  skill. 


OPTICAL    INSTRUMENTS.  177 

the  distance  at  which  the  clearest  vision  occurs  is  less  than 
ten  or  twelve  inches,  the  person  is  near-sighted,  and  when 
greater,  far-sighted.  Too  great  flatness  or  convexity  of  the 
cornea  or  crystalline  lens  will  produce  this  result.  The 
defect,  however,  often  lies  in  the  shape  of  the  eyeball.  In 
far-sightedness  the  ball  is  too  flat,  and  the  retina  too  near 
the  lens ;  in  near-sightedness  the  ball  is  elongated,  so  that 
the  retina  is  too  distant.  The  former  can  be  remedied  by 
convex  glasses,  which  bring  the  rays  to  a  focus  sooner,  and 
the  latter  by  concave,  which  throw  the  focus  further  back. 

The  retina  retains  an  impression  about  one-eighth  of  a 
second.  *  This  explains  why  a  wheel,  when  rapidly  revolved, 
appears  solid,  or  a  lighted  brand  like  a  ring  of  fire.  On  the 
other  hand,  it  requires  a  moment  for  an  impression  to  be 
made.  Thus  a  wheel  may  be  whirled  so  swiftly  that  its 
spokes  become  invisible. 

PRACTICAL  QUESTIONS.— 1.  Why  is  the  secondary  bow  fainter  than  the 
primary  ?  Why  are  the  colors  reversed  ?  2.  Why  can  we  not  see  around  the  corner 
of  a  house,  or  through  a  bent  tube  ?  3.  What  color  would  a  painter  use  if  he  wished 
to  represent  an  opening  into  a  dark  cellar  ?  4.  Is  white  a  color  ?  Is  black  ?  5.  By 
holding  an  object  nearer  a  light,  will  it  increase  or  diminish  the  size  of  the  shadow  ? 
6.  What  must  be  the  size  of  a  glass  in  order  to  reflect  a  full-length  image  of  a  person  ? 
Ans.  Half  the  person's  height.  7.  Where  should  we  look  for  a  rainbow  in  the 
morning  ?  8.  Can  two  spectators  see  the  same  bow  ?  9.  Why,  when  the  drops  of 
water  are  falling  through  the  air,  does  the  rainbow  appear  stationary  ?  10.  Why  can 
a  cat  see  in  the  night  ?  11.  Why  cannot  an  owl  see  in  daylight  ?  12.  Why  are  we 
blinded  when  we  pass  quickly  from  a  dark  into  a  lighted  room  ?  13.  If  the  light  of 
the  sun  upon  a  distant  planet  is  ^  of  that  which  we  receive,  how  does  its  distance 
from  the  sun  compare  with  ours  ?  14.  If,  when  I  sit  six  feet  from  a  candle,  I  receive 
a  certain  amount  of  light,  how  much  shall  I  diminish  it  if  I  move  back  six  feet 
further  ?  15.  Why  do  drops  of  rain,  in  falling,  appear  like  liquid  threads  ?  16.  Why 
does  a  towel  turn  darker  when  wet  ?  17.  Does  color  exist  in  the  object,  or  in  the 
mind  of  the  observer  ?  18.  Why  is  lather  opaque,  while  air  and  a  solution  of  soap 
are  each  transparent  ?  19.  Why  does  it  whiten  molasses  candy  to  u  pull  it "  f  20. 
Why  does  plastering  become  lighter  in  color  as  it  dries  ?  21.  Why  does  the  pho- 
tographer use  a  kerosene-oil  lamp  in  the  ll  dark  room  "  ?  22.  Is  the  common  division 
of  colors  into  "cold  "  and  "warm"  verified  in  philosophy?  23.  Why  is  the  image 
on  the  camera,  Fig.  175,  inverted '{  24.  Why  is  the  second  image  seen  in  a  mirror, 
Fig.  138,  brighter  than  the  first  ?  25.  Why  does  a  blow  on  the  head  make  one  u  see 
stars  "  1  Ans.  The  blow  excites  the  optic  nerve,  and  so  produces  the  sensation  of 
light.  26.  What  is  the  principle  of  the  kaleidoscope  ?  27.  Which  can  be  seen  at 

*  When  one  is  riding  slowly  on  the  cars  and  looking  at  the  landscape  between  the 
upright  fence-boards,  he  catches  only  glimpses  of  the  view;  but  when  moving 
rapidly,  these  snatches  uoitt  combine  to  form  a  perfect  landscape,  which  has,  however. 
a  grayish  tint,  owing  to  the  decreased  amount  of  light  reflected  to  the  eye. 


178  OPTICS. 

the  greater  distance— gray  or  yellow  ?  28.  Look  down  Into  the  glass  of  water  shown 
in  Pig.  145,  and,  at  a  certain  angle,  you  will  see  two  spoons,  one  small  and  having 
the  real  handle  of  the  spoon,  though  apparently  bent,  and  the  real  spoon  with  no 
handle.  Explain.  29.  When  a  star  is  near  the  horizon,  does  it  seem  higher  or  lower 
than  its  true  place  ?  30.  Why  can  we  not  see  a  rainbow  at  midday  ?  31.  What  con- 
clusion do  we  draw  from  the  fact  that  moonlight  shows  the  same  dark  lines  as  sun- 
light ?  32.  Why  does  the  bottom  of  a  ship  seen  under  water  appear  flatter  than  it 
really  is  ?  33.  Of  what  shape  does  a  round  body  appear  in  water  ?  34.  Why  is  rough 
glass  translucent  while  smooth  glass  is  transparent?  85.  Why  are  some  bodies 
brilliant  and  others  dull  ?  36.  Why  can  a  carpenter,  by  looking  along  the  edge  of  a 
board,  tell  whether  it  is  straight  ?  88.  Why  can  we  not  see  out  of  the  window 
after  we  have  lighted  the  lamp  in  the  evening  ?  38.  Why  does  a  ground-glass  globe 
soften  the  light  ?  39.  Why  can  we  not  see  through  ground-glass  or  paiutod  windows  ? 
40.  Why  does  the  moon's  surface  appear  flat?  41.  Why  can  we  see  farther  with  a 
telescope  than  with  the  naked  eye  ?  42.  Why  is  not  snow  transparent,  like  ice  ? 
43.  Are  there  rays  hi  the  sunbeam  which  we  cannot  perceive  with  the  eye  ?  44  Why, 
when  we  press  the  finger  on  one  eyeball,  do  we  see  objects  double  ?  45.  Why  does 
a  distant  light,  in  the  night,  seem  like  a  star?  46.  Why  does  R  bright  light,  in  the 
night,  seem  so  much  nearer  than  it  is  ?  47.  What  color  predominates  in  artificial 
lights  ?  Ans.  Yellow.  48.  Why  does  yellow  seem  white,  and  blue  green,  when  seen 
by  artificial  light  ?  49.  Why  are  we  not  sensible  of  darkness  when  we  wink?  50. 
Why  is  the  lens  of  a  fish's  eye  (seen  in  the  eye-socket  of  a  boiled  fish)  so  convex  ? 

51.  When  do  the  eyes  of  a  portrait  seem  to  follow  a  spectator  to  all  parts  of  a  room  ? 

52.  Why  does  the  dome  of  the  sky  seem  flattened  ?    53.  Why  do  the  two  parallel 
tracks  of  a  railroad  appear  to  approach  in  the  distance  ?    54.  Why  does  a  fog  mag- 
nify objects  ?    55.  If  you  sit  where  you  cannot  see  another  person's  image,  why 
cannot  that  person  see  yours  ?    56.  Why  can  we  see  the  multiple  images  in  a 
mirror  better  if  we  look  into  it  very  obliquely  ?    57.  Why  is  an  image  seen  in  water 
inverted?    58.  Why  is  the  sun's  light  fainter  at  sunset  than  at  midday?    59.  Why 
can  we  not  see  the  fence-posts  when  we  are  riding  rapidly?    60.  Ought  a  red  flower 
to  be  placed  in  a  bouquet  by  an  orange  one  ?    A  pink  or  blue  with  a  violet  one  ? 
61.  Why  are  the  clouds  white  while  the  clear  sky  is  blue?    62.  Why  does  skim- 
milk  look  blue  and  new  milk  white  ?    63.  What  would  be  the  effect  of  filling  the 
basin,  in  the  experiment  shown  in  Fig.  147,  with  salt  water  ?    64.  Why  is  not  the 
image  of  the  sun  in  water  at  midday  so  bright  as  near  sunset  ?    65.  Why  is  the 
rainbow  always  opposite  the  sun  ? 


SUMMARY. 

Eight  comes  from  the  sun  and  other  self-luminous  bodies.  It  is 
transmitted  by  means  of  vibrations  in  ether,  according  to  the  principles 
of  wave-motion.  It  radiates  equally  in  all  directions,  travels  in 
straight  lines,  decreases  as  the  square  of  the  distance,  and  moves 
186,000  miles  per  second.  Light  falling  upon  a  body  may  be  absorbed, 
transmitted  or  reflected.  If  the  surface  be  rough,  the  irregularly- 
reflected  light  enables  us  to  see  the  body  ;  if  it  be  smooth  and  highly 
polished,  the  rays  are  reflected  so  nearly  as  they  fall  that  they  form 
an  image  of  the  original  object.  Surfaces  producing  such  images  are 


SUMMARY.  179 

termed  mirrors — plane,  concave,  or  convex.  The  image  is  seen  in  the 
line  of  the  reflected  ray,  and,  in  a  plane  mirror,  as  far  behind  the 
mirror  as  the  object  is  in  front.  Multiple  images  are  produced  by 
repeated  reflections,  as  in  the  kaleidoscope.  A  concave  mirror,  as 
generally  used,  collects  the  rays,  and  serves  to  magnify  an  object  or  to 
throw  a  parallel  beam  of  light.  A  convex  mirror  scatters  the  rays, 
and  apparently  diminishes  the  size  of  an  object. 

When  a  ray  enters  or  leaves  a  transparent  body  obliquely  it  is 
refracted  ;  if  passing  into  a  rarer  medium  it  is  bent  from,  and  if  into 
a  denser,  toward  a  perpendicular.  A  transparent  body  with  one  or 
more  curved  surfaces  is  a  lens.  There  are  two  classes— convex  and 
concave.  The  former  lens,  as  generally  used,  tends,  like  a  concave 
mirror,  to  collect  the  rays  of  light,  and  is  known  as  a  "magnifier"; 
the  latter,  like  a  convex  mirror,  scatters  the  rays  of  light,  and  is 
known  as  a  "diminisher."  Mirage  is  an  optical  delusion  caused  by 
reflection  and  refraction  of  light  in  passing  through  air  composed  of 
strata  of  unequal  density.  Owing  to  the  varying  refrangibility  of  the 
different  constituents  of  the  sunbeam,  a  prism  can  disperse  them  into 
a  colored  band  called  the  solar  spectrum.  The  spectrum  shows  white 
light  to  consist  of  seven  elementary  colors,  and  that  the  sunbeam  con- 
tains, in  addition  to  the  luminous  rays,  heat  and  chemical  rays.  By 
means  of  the  spectroscope  we  can  examine  the  spectrum  of  a  flame, 
and  find  whether  it  is  a  burning  gas  or  an  incandescent  solid.  Each 
substance  gives  a  spectrum  with  its  peculiar  lines  of  color.  A  gas 
absorbs  the  same  rays  that  it  is  capable  of  emitting  ;  hence  we  have 
absorption  spectra,  which  contain  dark  instead  of  colored  lines.  A 
delicate  mode  of  analysis  is  thus  furnished,  whereby  the  elements 
even  of  the  distant  stars  can  be  detected.  The  rainbow  is  formed  by 
the  refraction  and  reflection  of  the  sunbeam  in  raindrops.  Light, 
when  reflected  by  or  transmitted  through  bodies,  is  so  modified,  chiefly 
by  absorption,  as  to  produce  the  varied  phenomena  of  color.  Each 
color  has  its  own  wave-length,  the  minuteness  of  which  is  almost 
incredible.  Different  systems  of  light,  as  of  sound  waves,  may  co- 
exist. But  if  any  two  coincide  with  similar  phases  they  will  strengthen 
each  other  ;  and  if  with  opposite  phases,  weaken  each  other.  Inter- 
ference of  light,  as  thus  produced,  causes  the  play  of  colors  in  the 
soap-bubble,  mother-of-pearl,  etc.  Polarized  light  is  that  in  which 
the  molecular  vibrations  are  made  in  the  same  plane.  It  is  of  use  in 
determining  the  internal  constitution  of  a  body. 

The  principal  optical  instruments,  including  the  eye,  are  adapted 
to  produce  and  examine  the  image  formed  by  a  lens.  In  the  magic- 
lantern,  stereopticon,  and  solar  microscope,  the  image  is  thrown  on  a 
screen  in  a  dark  room — lamplight  being  used  in  the  first,  the  calcium 
light  in  the  second,  and  sunlight  in  the  third.  In  the  refracting  tele- 


180  OPTICS. 

scope  and  the  microscope,  the  image  is  formed  in  a  tube  Toy  a  lens  at 
one  end  and  looked  at  from  behind  by  a  lens  at  the  other  end.  In  the 
eye,  which  is  a  small  camera-obscura,  the  image  is  formed  on  the 
retina,  whence  the  sensation  is  carried  by  the  optic  nerve  to  the 
brain. 


HISTORICAL    SKETCH. 

The  ancients  knew  that  light  is  propagated  in  straight  lines.  They 
deduced  the  laws  of  reflection,  and  we  read  that  Archimedes  set  fire  to 
the  Roman  ships  off  Syracuse  by  means  of  concave  mirrors.  Euclid 
and  Plato,  however,  thought  that  the  ray  of  light  proceeds  from  the 
eye  to  the  object,  an  error  that  was  long  of  correction.  One  thousand 
years  did  not  pursue  the  subject  into  other  departments.  The  Arabian 
philosopher,  Alhazen,  who  lived  in  the  eleventh  century,  discovered 
the  phenomenon  shown  in  Fig.  147.  About  1608  the  telescope  was 
invented  by  the  Dutch.*  Jansen,  Metius  and  Lippersheim  each  claimed 
the  honor,  and  the  legend  is  that  the  discovery  grew  out  of  some 
children  at  play,  accidentally  arranging  two  watch-glasses  so  as  to 
magnify  a  distant  object.  In  fact,  however,  the  action  of  the  convex 
lens  was  already  known,  the  compound  microscope  had  been  in- 
vented by  Jansen  20  years  previously,  and  the  simple  microscope  was 
known  to  the  ancient  Chaldeans.  In  1621  Snell  discovered  the  law  of 
refraction.  By  its  aid  Descartes  explained  the  rainbow.  Half  a  cen- 
tury of  waiting,  and  Newton  published  his  investigations  in  the 
decomposition  of  light.  He,  however,  believed  in  what  is  known  as 
the  "corpuscular  theory,"  that  light  consists  of  minute  particles  of  mat- 
ter radiated  in  straight  lines  from  a  luminous  object.  In  1676  Roemer, 
by  observing  Jupiter's  moons  (p.  150),  found  out  the  velocity  of  light, 
which  up  to  that  time  had  been  considered  instantaneous.  A  little 
later,  Huygens  advanced  the  undulatory  theory,  which  was  applied 
with  singular  skill  by  Young  and  Fresnel,  in  the  first  quarter  of  the 
present  century,  to  explain  all  optical  phenomena.  (See  list  of  books 
for  additional  information,  on  p.  120.) 

*  "  In  1609,  the  government  of  Venice  made  a  considerable  present  to  Signer 
Galileo,  of  Florence,  Professor  of  Mathematics  at  Padua,  and  increased  his  annual 
stipend  by  100  crowns,  because,  with  diligent  study,  he  found  out  a  rule  and  measure 
by  which  it  is  possible  to  see  places  30  miles  distant  as  if  they  were  near,  and,  on  the 
other  hand,  near  objects  to  appear  much  larger  than  they  are  before  our  eyes." — 
From  an  old  paper  in  the  Library  of  Heidelberg  University. 


VIII. 


"  The  combustion  of  a  single  pound  of  coal,  supposing  it  to  take  place  in 
a  minute,  is  equivalent  to  the  work  of  three  hundred  horses  ;  and  the  force 
set  free  in  the  burning  of  300  Ibs.  of  coal  is  equivalent  to  the  work  of  an 
able-bodied  man  for  a  lifetime" 


ANALYSIS. 


EH' 
<J 

w 
w 


PRODUCTION 
HEAT. 


OF 


2.   PHYSICAL   EFFECTS 
OF   HEAT. 


3. 


COMMUNICATION 
OF   HEAT. 


4.     STEAM-ENGINE. 


5.     METEOROLOGY. 


1.  DEFINITIONS. 

2.  RELATION  BETWEEN  LIGHT  AND 

HEAT. 

3.  THEORY  OF  HEAT. 

4.  SOURCES  OF  HEAT. 

5.  MECHANICAL  EQUIVALENT  OF 

HEAT. 

1.  TEMPERATURE     AND     SPECIFIC 

HEAT. 

2.  EXPANSION. 

3.  LIQUEFACTION. 

4.  VAPORIZATION. 

5.  EVAPORATION. 

6.  SPHEROIDAL  STATE. 


CONDUCTION. 

3.   CONVECTION 

3.  RADIATION. 


(2.)  Of  Gases. 


1.  THEORY  OF  STEAM-ENGINE. 

2.  GOVERNOR. 

3.  HIGH  PRESSURE  ENGINE. 

1.  GENERAL  PRINCIPLES. 

2.  DEW. 

3.  FOGS. 

4.  CLOUDS. 

5.  RAIN. 

6.  WINDS. 

7.  OCEAN  CURRENTS. 

8.  ADAPTATIONS  OF  WATEB. 


HEAT. 

1.—  PRODUCTION  OF  HEAT. 

1.  Definitions.  —  Luminous  heat  is  that  which  radiates 
from  a  luminous  body.     Ex.  :  The  heat  of  a  white-hot  iron. 
Obscure  heat  is  that  from  a  non-luminous  source.     Cold  is  a 
relative  term,  indicating  the  partial  absence  of  heat.     Gases 
and  Vapors  differ  but  slightly.     The  former  retain  their 
form  at  the  ordinary  temperature  and  pressure.     Ex.  :  Air. 
The  latter  are  readily  condensed  and  at  the  ordinary  tem- 
perature appear  as  liquids  or  solids.     Ex.  :  Steam. 

2.  Relation  between  Light  and  Heat.  —  Thrust  a 
cold  iron  into  the  fire.     It  is  at  first  dark,  but  soon  becomes 
luminous,  like  the  glowing  coals.  —  Eaise  the  temperature  of 
a  platinum  wire.     We  quickly  feel  the  radiation  of  obscure 
heat-rays.     As  the  metal  begins  to  glow,  our  eyes  detect  a 
red  color,  then  orange  combined  with  it,  and  so  on  through 
the  spectrum.     At  last  all  the  colors  are  emitted,  and  the 
metal  is  dazzling  white.     All  bodies  become  luminous  at  a 
fixed  temperature.     Like  light,  heat  may  be  reflected,  re- 
fracted, and  polarized.      It  radiates  in  straight  lines  in 
every  direction,  and  decreases  in  intensity  as  the  square  of 
the  distance.     It  movjes  with  ,th 


is  therefore  believed  that  light  is  luminous  heat,  and  that 
the  three  classes  of  waves  in  the  solar  spectrum  differ  merely 
as  one  color  from  another,  in  the  rapidity  of  the  vibrations.* 

*  According  to  Tyndall,  95  per  cent,  of  the  rays  from  a  candle  are  invisible  or  heat- 
rays.  These  may  be  brought  to  a  focus  and  bodies  fired  in  the  darkness.—  Each  of 
the  five  classes  of  nerves  seems  to  be  adapted  to  transmit  vibrations  of  its  own  kind, 
while  it  is  insensible  to  the  others.  Thus,  if  the  rate  of  oscillation  be  less  than  that 
of  red,  or  more  than  that  of  violet,  the  optic  nerve  is  uninfluenced  by  the  waves.  We 
cannot  see  with  our  fingers,  taste  with  our  ears,  or  hear  with  our  nose.  Yet  these 
are  organs  of  sensation  and  sensitive  to  their  peculiar  impressions.—1'  Suppose,  by 
a  wild  stretch  of  imagination,  some  mechanism  that  will  make  a  rod  turn  round  one 
of  its  ends,  quite  slowly  at  first,  but  then  faster  and  faster,  till  it  will  revolve  any 
number  of  times  in  a  second  ,  which  is,  of  course,  perfectly  imaginable,  though  you 


184  HEAT. 

The  longer  and  slower  waves  of  ether  fall  upon  the  nerves 
of  touch,  and  produce  the  sensation  of  heat.  The  more 
rapid  affect  the  optic  nerve  and  produce  the  sensation  of 
light.  The  shortest  and  quickest  cause  chemical  changes. 

3.  Theory  of  Heat. — Heat  is  motion.  The  molecules 
of  a  solid  are  in  constant  vibration.  When  we  increase  the 
rapidity  of  this  oscillation,  we  heat  the  body;  when  we 
decrease  it,  we  cool  the  body.  The  vacant  spaces  between 
the  molecules  are  filled  with  ether.  As  the  air  moving 
among  the  limbs  of  a  tree  sets  its  boughs  in  motion,  and  in 
turn  is  kept  in  motion  by  the  waving  branches,  so  the 
ether  puts  the  molecules  in  vibration,  or  is  thrown  into 
vibration  by  them.  Ex. :  Insert  one  end  of  a  poker  in  the  fire. 
The  particles  immersed  in  the  fire  are  made  to  vibrate  in- 
tensely; the  swinging  molecules  strike  their  neighbors,  and 
so  on  continually,  until  the  oscillation  reaches  the  other 
end.  If  we  handle  the  poker,  the  motion  is  imparted  to  the 
delicate  nerves  of  touch ;  they  carry  it  to  the  brain,  and 
pain  is  felt.  In  popular  language,  "the  iron  is  hot,"  and 

could  not  find  such  a  rod  or  put  together  such  a  mechanism.  Let  the  whirling  go  on 
In  a  dark  room,  and  suppose  a  man  there  knowing  nothing  of  the  rod  ;  how  will  he 
be  affected  hy  it  ?  So  long  as  it  turns  but  a  few  tunes  in  the  second,  he  will  not  be 
affected  at  all  unless  he  is  near  enough  to  receive  a  blow  on  the  skin.  But  as  soon  as 
it  begins  to  spin  from  sixteen  to  twenty  times  a  second,  a  deep  growling  note  will 
break  in  upon  him  through  his  ear ;  and  as  the  rate  then  grows  swifter,  the  tone  will 
go  on  becoming  less  and  less  grave,  and  soon  more  and  more  acute,  till  it  will  reach  a 
pitch  of  shrillness  hardly  to  be  borne,  when  the  speed  has  to  be  counted  by  tens  of 
thousands.  At  length,  about  the  stage  of  forty  thousand  revolutions  a  second,  more 
or  less,  the  shrillness  will  pass  into  stillness  ;  silence  will  again  reign  as  at  first,  nor 
any  more  be  broken.  The  rod  might  now  plunge  on  in  mad  fury  for  a  long  time  with- 
out making  any  difference  to  the  man  ;  but  let  it  suddenly  come  to  whirl  some  million 
times  a  second,  and  then  through  intervening  space  faint  rays  of  heat  will  begin  to 
steal  towards  him,  setting  up  a  feeling  of  warmth  in  his  skin ;  which  again  will  grow 
more  and  more  intense,  as  now  through  tens  and  hundreds  and  thousands  of  millions 
the  rate  of  revolution  is  supposed  to  rise.  Why  not  billions  ?  The  heat  at  first  will 
be  only  so  much  the  greater.  But,  lo !  about  the  stage  of  four  hundred  billions  there 
is  more— a  dim  red  .ight  becomes  visible  in  the  gloom ;  and  now,  while  the  rate  still 
mounts  up,  the  heat  in  its  turn  dies  away,  till  it  vanishes  as  the  sound  vanished ; 
but  the  red  light  will  have  passed  for  the  eye  into  a  yellow,  a  green,  a  blue,  and,  last 
of  all,  a  violet.  And  to  the  violet,  the  revolutions  being  now  about  eight  hundred 
billions  a  second,  there  will  succeed  darkness— night,  as  in  the  beginning.  This 
darkness  too,  like  the  stillness,  will  never  more  be  broken.  Let  the  rod  whirl  on  at 
}  t  may,  its  doings  cannot  come  within  the  ken  of  that  man's  senses/* 


PRODUCTION    OF    HEAT.  185 

we  are  burned.  If,  without  touching  it,  we  hold  our  hand 
near  the  poker,  the  ether-waves  set  in  motion  hy  the  mole- 
cules of  iron  strike  against  the  hand,  and  produce  a  less 
intense  sensation  of  heat.  In  the  former  case,  the  fierce 
motion  is  imparted  directly;  in  the  latter,  the  ether  acts  as 
a  carrier  to  bring  it  to  us. 

4.  The  Sources  of  Heat  are  the  sun,  stars,  and  me- 
chanical and  chemical  forces. 

(1.)  The  molecules  of  the  sun  and  stars  are  in  rapid  vibra- 
tion. These  set  in  motion  waves  of  ether,  which  dart  across 
the  intervening  space,  and  surging  against  the  earth,  give 
up  their  motion  to  it.  (2. )  Friction  and  percussion  produce 
heat,  because  additional  motion  is  thereby  imparted  to  the 
vibrating  particles.*  (3.)  Chemical  action  is  seen  in  fire. 
The  oxygen  of  the  air  has  an  affinity  for  the  carbon  and 
hydrogen  of  the  fuel.  They  rush  together.  As  they  strike, 
their  motion  is  stopped.  The  shock  sets  the  molecules  in 
vibration.  They  impart  their  motion  to  the  ether,  and 
thus  start  waves  of  heat. 

5.  Mechanical  Equivalent  of  Heat  (Joule's  Law). — 
In  these  various  changes  of  mechanical-motion  into  heat- 
motion  no  energy  is  lost.    If  the  heat  produced  by  the  black- 
smith's hammer  falling  on  the  anvil  could  be  gathered  up, 
it  would  be  sufficient  to  lift  the  hammer  to  the  point  from 
which  it  fell.     A  pound-weight  falling  772  feet,  ivill  generate 
enough  heat  to  raise  the  temperature  of  1  Ib.  of  water  1°; 

*  Savages  obtain  fire  by  rubbing  together  two  pieces  of  wood.— A  horse  hits  his 
ehoes  against  a  stone  and  "  strikes  fire ; "  little  particles  of  the  metal  being  torn  off 
are  heated  by  the  shock  and  bum  as  sparks.— A  bullet  checked  instantly,  as  by  a 
bone  in  the  body,  is  partially  fused.— A  train  of  cars  is  stopped  by  the  pressure  of  the 
brakes.  In  a  dark  night,  we  see  the  sparks  flying  from  the  wheels,  the  motion  of 
the  train  being  converted  into  heat.— A  blacksmith  pounds  a  piece  of  iron  until 
it  glows.  His  strokes  set  the  molecules  of  met;:!  vibrating  rapidly  enough  to  send 
ether-waves  of  such  swiftness  as  to  affect  the  eye  of  the  observer.— As  a  cannon-shot 
strikes  an  iron  target,  a  group  of  sparks  pours  from  it.— Were  the  earth  instantly 
stopped,  enough  heat  would  be  produced  to  "  raise  a  lead  ball  the  size  of  our  globe 
to  884,000°  C."  If  it  were  to  fall  to  the  sun  its  impact  would  produce  a  thousand 
times  mere  heat  than  its  burning.  The  earth  thus  contains  within  itself  the  elements 
for  the  fulfillment  of  the  prediction  that  it  shall  "  melt  wilh  fervent  heat." 


186 

conversely,  the  amount  of  heat  necessary  to  elevate  the 
temperature  of  1  Ib.  of  water  1°?  will  raise  a  pound-weight 
772  feet. 

2.    PHYSICAL    EFFECTS    OF    HEAT. 

1.  Temperature. — The  heat-force  increases  the  kinetic 
energy  (vis  viva,  p.  37)  of  the  molecules  and  so  elevates  the 
temperature  of  a  body.     If,  however,  the  same  amount  of 
heat  be  applied  to  the  same  weight  of  different  substances 
they  will  not  show  the  same  increase  of  temperature.     In 
estimating  the  specific  heat  (see  specific  gravity,  p.  92)  of 
the  various  kinds  of  matter,  the  quantity  of  heat  required 
to  raise  the  temperature  of  1  Ib.  of  water  1°  is  taken  as  the 
standard.     That  amount  would  elevate  the  temperature  of 
1  Ib.  of  mercury  30°;  hence  its  specific  heat  is  -fa. 

2.  Expansion. — The  heat-force  urges  the  molecules  of  a 
body  into  longer  vibrations  and  so  increases  its  size.     Hence 
the  general  law  "Heat  expands  and  cold  contracts."    As  a 
rule,  gases  expand  most,  liquids  next  and  solids  least.     The 
expansive  force  exerted  is  often  enormous.     Thus  a  rise  of 
45°  C.  in  the  temperature,  which  may  occur  during  a  sum- 
mer's day,  will  lengthen  a  bar  of  wrought-iron,  10  inches 
long,  -g-j-g-  of  an  inch,  and  if  the  ends  are  fastened,  exert  a 
strain  of  50  tons.     When  the  metal  cools,  it  will  contract 
with  the  same  force.* 

The  Mercurial  Thermometer  is  an  instrument  for  measur- 
ing temperature  by  the  expansion  of  mercury,  f    To  graduate 

*  A  carriage-tire  is  put  on  when  hot,  that,  when  cooled,  it  may  bind  the  wheel 
together.— Rivets  used  in  fastening  the  plates  of  steam-boilers  are  inserted  red-hot. 
— "  The  ponderous  iron  tubes  of  the  Britannia  bridge  writhe  and  twist,  like  a  huge 
serpent,  under  the  varying  influence  of  the  solar  heat.  A  span  of  the  tube  is  de- 
pressed only  a  quarter  of  an  inch  by  the  heaviest  train  of  cars,  while  the  sun  lifts  it 
2|  inches."— The  Bunker-hill  monument  nods  as  it  follows  the  sun  in  its  daily 
course.— Tumblers  of  thick  glass  break  on  the  sudden  application  of  heat,  because 
the  surface  dilates  before  the  motion  has  time  to  reach  the  interior. 

+  Take  the  glass  tube  shown  in  Fig.  68,  and  heat  the  bulb  to  expel  the  air,  Then 
plunge  the  stem  in  colored  water.  As  the  bulb  cools,  the  water  will  rise  and  partly 
fill  it.  Heat  the  bulb  again  until  the  steam  pours  out  of  the  stem.  On  inserting  it  a 
second  time,  the  water  will  fill  the  bulb.  In  the  manufacture  of  thermometers,  it  is 
customary  to  have  a  cup  blown  at  the  upper  end  of  the  stem.  This  is  filled  with 
mercury,  and  the  air,  when  expanded,  bubbles  out  through  it,  while  the  metal 


PHYSICAL    EFFECTS    OF    HEAT. 


18? 


it,  according  to  Fahrenheit's  scale  (F.),  each  thermometer  is 
put  in  melting  ice,  and  the  point  to  which  the  mercury 
sinks  is  marked  32°,  Freezing-point.*     It  is 
then  placed  in  a  steam-bath,  and  the  point  to        Fia- m- 
which  the  mercury  rises  (when  the  barometric 
column  stands  at  30  in.)  is  marked  212°,  Boil- 
ing-point.    The  space  between  these  two  points 
is  divided  into  180°.     In  the  Centigrade  scale 
(0.)  the  freezing-point  is  0,  and  the  boiling- 
point  100°.     In  Eeaumur's  scale  (R.),  the  boil- 
ing point  is  80°.  f 

3.  Liquefaction  or  fusion.  "When  heat  is 
applied  to  a  solid  body,  a  point  is  finally  reached 
when  this  repellent  force  neutralizes  the  attrac- 
tive force  (p.  43).  The  molecules  then,  escap- 
ing the  grasp  of  cohesion,  move  freely  on  one 
another.  In  this  process,  large  quantities  of 
heat  are  consumed.  Thus  if  ice  at  32°  be 
melted,  1^2°  of  heat  will  disappear  J  and  the 
water  will  still  indicate  only  32°.  Heat  which 
thus  enters  a  body  without  raising  its  temperature,  is  termed 
latent  heat.  § 

trickles  down  as  the  bulb  cools.  The  mercury  is  then  highly  heated,  when  the  tube 
is  melted  off  and  sealed  at  the  end  of  the  column  of  mercury.  The  metal  contracts 
on  cooling,  and  leaves  a  vacuum  above. 

*  The  inventor  placed  zero  32°  below  the  temperature  of  freezing  water,  because 
he  thought  that  absolute  cold — a  point  now  estimated  to  be  about  —273°  C. 

t  The  following  formulae  will  be  of  use  in  comparing  the  readings  of  the  different 

Bcales :  R  =  |  C  =  |(F  —  32°).  (1.) 

C  =  |  R  =r  f  (F  -  32°).  (2.) 

F  =  I  C  +  32°  =  1  R  +  32°.  (3.) 

1°  C  =  1.8°  F.  (4.) 

\  This  fact  explains  why  it  takes  so  long  a  time  and  so  hot  a  fire  to  melt  snow,  and 
the  water  when  formed  is  yet  ice-cold. 

§  It  must  not  be  supposed  when  considering  the  various  changes  of  solids  to 
liquids,  liquids  to  gases,  etc.,  that  the  sensible  heat  which  becomes  latent  is  lost.  It 
is  occupied  in  doing  work,  as  in  neutralizing  the  force  of  cohesion  and  in  overcoming 
the  pressure  of  the  air  which  opposes  expansion.  The  heat-force  thereby  becomes  a 
potential  energy  which  can  be  converted  into  kinetic.  When  steam,  vapor,  gas,  and 
liquids  pass  back  into  their  former  state,  their  latent  heat  is  restored  as  sensible. 
We  thus  reach  the  paradoxical  conclusion  that  thawing  is  a  cooling  process  and 
freezing  is  a  warming  process. 


188 


HEAT. 


Freezing  mixtures  depend  on  the  principle  just  explained. 

In  freezing  ice-cream,  salt  and  ice  are  used.  Salt  haying  an 
attraction  for  water  dissolves  the  ice,  and  then  itself  dissolves 
in  the  water  thus  formed.  In  this  process  two  solids  become 
liquids.  The  necessary  heat  is  absorbed  mainly  from  the 
cream. 

Liquefaction  of  gases.  When  a  gas  is  cooled,  the  repellent 
force  is  weakened,  and  the  molecules  once  more  approach 
one  another.  By  the  continued  action  of  cold  and  pressure 
every  known  gas  can  be  reduced  to  a  liquid  form.  In  the 
case  of  carbonic  acid,  nitrogen,  oxygen,  etc.,  the  instant  the 
pressure  is  removed  they  resume  the  gaseous  state. 

FIG  178. 


4.  Vaporization. — When  heat  is  applied  to  a  liquid  the 
temperature  rises  until  the  boiling-point  is  reached,  when  it 
stops  and  the  liquid  is  changed  to  vapor  at  that  constant 
temperature.  The  vapor  is  nearly  free  from  solids  dissolved 
in  the  liquid.  Ex.:  Pure  or  distilled  water  is  obtained 
by  heating  water  in  a  boiler  A,  whence  the  steam  passes 


PHYSICAL    EFFECTS    OF    HEAT.  189 

through  the  pipe  0  and  the  worm  within  the  condenser  S, 
where  it  is  condensed  and  drops  into  the  vessel  D.  The 
pipe  is  coiled  in  a  spiral  form  within  the  condenser,  and  is 
hence  termed  the  worm.  The  condenser  is  kept  full  of  cold 
water  from  the  tub  at  the  left.  By  carefully  regulating  the 
heat,  one  liquid  may  be  separated  from  another  by  distilla- 
tion. (See  Chemistry,  p.  196.) 

Boiling-point.  When  we  heat  water,  the  bubbles  which 
pass  off  first  are  the  air  dissolved  in  the  liquid  ;  next  bubbles 
of  steam  form  on  the  bottom  and  sides  of  the  vessel,  and, 
rising  a  little  distance,  are  condensed  by  the  cold  water. 
Collapsing,  they  produce  the  sound  known  as  "simmering." 
As  the  temperature  of  the  water  rises,  they  ascend  higher, 
until  they  burst  at  the  surface,  and  pass  off  into  the  air. 
The  violent  agitation  of  the  water  thus  produced  is  termed 
boiling.*  The  boiling-point  is  not  the  same  in  different 
liquids.  This  causes  the  variety  in  the  forms  of  matter. 
Some  substances  vaporize  at  ordinary  temperatures  ;  others 
melt  only  at  the  highest ;  while  the  gases  of  the  air  are  but 
products  from  substances  which  vaporize  at  enormously  low 
temperatures. 

The  boiling-point  of  water  depends  on  three  circum- 
stances:  (1.)  Purify  of  the  water.  A  substance  dissolved 
in  water  ordinarily  elevates  the  boiling-point.  Thus  salt 
water  boils  at  a  higher  temperature  than  pure  water.  The 
air  dissolved  in  water  tends  by  its  elastic  force  to  separate 
the  molecules.  If  this  be  removed,  the  boiling-point  may 

*  The  temperature  of  water  cannot  be  raised  above  the  boiling-point,  unless  the 
steam  be  confined.  The  extra  force  is  occupied  in  expanding  the  water  into  steam. 
This  occupies  1,700  times  the  space,  and  is  of  the  same  temperature  as  the  water  from 
which  it  is  made.  Nearly  1000°  of  heat  become  latent  in  this  process,  but  are  made 
sensible  again  when  the  steam  is  condensed.  Water  requires  more  heat  to  warm  it 
and  gives  out  more  heat  when  it  cools  (i.  e.,  it  has  a  higher  specific  heat)  than,  save 
one  or  two  unimportant  exceptions,  any  other  substance.  When  ice  is  at  32°  it  must 
absorb  142°  before  it  becomes  water  at  32° ;  and  when  water  at  32°  it  must  give  off 
142°  before  it  solidifies.  The  heat  which  boils  ice-cold  water  would  raise  iron  to  a 
glowing  red.  "  The  heat-force  required  to  turn  a  pound  of  water  at  32°  into  steam 
would  lift  a  ton  weight  nearly  400  feet  high."  Steam  is  invisible.  This  we  can  verify 
by  examining  it  where  it  issues  from  the  spout  of  the  tea-kettle.  It  soon  condenses, 
however,  into  minute  globules,  which,  floating  in  the  true  steam,  render  the  vapor 
apparently  visible. 


190 


HEAT. 


FIG.  179. 


be  elevated  over  50°,  when  the  water  will  be  converted  into 
steam  with  explosive  violence. 

(2.)  Nature  of  the  vessel.  Water  will  boil  at  a  lower 
temperature  in  iron  than  in  glass.  When  the  surface  of  the 
glass  is  chemically  clean,  the  boiling-point  is  still  higher. 
This  seems  to  depend  on  the  strength  oj_the  adhesion 
between  the  water  and  the  containing  vessel. 

(3.)  Pressure  upon  the  surface  raises  the  boiling-point.* 
Water,  therefore,  boils  at  a  lower  temperature  on  a  moun- 
tain than  in  a  valley.  The 
temperature  of  boiling  water  at 
Quito  is  90°  0.,  and  on  Mont 
Blanc  84°.  The  variation  is  so 
uniform  that  the  height  of  a 
place  can  thus  be  ascertained ; 
an  ascent  of  596  feet  producing 
a  difference  of  1°  F. 

.The  influence  of  pressure  is 
well  illustrated  by  the  following 
experiment :  Boil  a  glass  flask 
half  full  of  water  so  as  to  ex- 
pel the  air.  Cork  quickly  and 
invert.  The  pressure  of  the 
steam  will  stop  ebullition.  A 
few  drops  of  cold  water  will 
condense  the  steam,  and  boiling 
will  re-commence.  This  will 
soon  be  checked,  but  can  be 
restored  as  before.  The  process  may  be  repeated  until  the 
water  cools  to  blood-heat.  The  cushion  of  air  which  com- 
monly breaks  the  fall  of  water  is  removed,  and  if  the  cork 
be  air-tight,  the  water,  when  cold,  will  strike  against  the 
flask  with  a  sharp,  metallic  sound. 

*  Preesure  opposes  the  repellent  heat-force,  and  so  renders  it  easier  for  cohesion 
to  hold  the  particles  together.  In  the  interior  of  the  earth  there  may  be  masses  of 
matter  heated  red  or  white-hot  and  yet  solid,  more  rigid  even  than  glass,  in  conse- 
quence of  their  melting  point  being  raised  BO  high  by  the  tremendous  pressure  that 
they  cannot  liquefy.— ToAL 


PHYSICAL    EFFECTS    OF    HEAT. 


191 


5.  Evaporation  is  a  slow  formation  of  vapor,  which 
takes  place  at  ordinary  temperatures.  Water  evaporates 
even  at  the  freezing-point.  Clothes  dry  in  the  open  air 
in  the  coldest  weather.  The  wind  quickens  the  process, 
because  it  drives  away  the  moist  air  near  the  clothes  and 
supplies  dry  air.  Evaporation  is  also  hastened  by  an  in- 
crease of  surface  and  a  gentle  heat. 

Vacuum  pans  are  employed  in  condensing  milk  and  in 
the  manufacture  of  sugar.  They  are  so  arranged  that  the 
air  above  the  liquid  in  the  vessel  may  be  exhausted,  and 
then  the  evaporation  takes  place  rapidly,  and  at  so  low  a 
temperature  that  burning  is  avoided. 

The  cooling  effect  of  evaporation  is  due  to  sensible  heat 
becoming  latent  in  the  vapor.*  Water  may  be  frozen  in  a 
vacuum,  if  the  vapor  be  removed  as  fast  as  formed,  f  Ice 
is  manufactured  in  the  tropics  by  machines  constructed  on 
this  principle.  The  greatest  artificial  cold  known,  —220°  F. , 
was  produced  by  evaporating  in  a  vacuum  liquid  nitrous 
oxide  gas  and  disulphide  of  carbon. 


*  Pious   Mohammedans  were  formerly  ac-  FIG.  180. 

customed  to  place,  In  niches  along  the  public 
streets,  porous  earthenware  bottles  (Fig.  180), 
filled  with  water,  to  refresh  the  thirsty  travel- 
lers. 

t  There  is  an  apparatus  for  freezing  water 
under  the  receiver  of  an  air-pump.  A  watch- 
glass  of  water  is  placed  over  a  pan  of  strong 
eulphuric  acid,  which  absorbs  the  vapor,  and, 
in  the  vacuum,  a  part  of  the  water  evaporates 
so  rapidly  as  to  freeze  the  remainder.— Liquid 
carbonic  acid  exposed  to  the  open  air  evaporates 
so  quickly  as  to  convert  itself  into  a  snowy- 
white  solid.  This  will  solidify  mercury.  On 
throwing  the  frozen  metal  into  water,  the  mer- 
cury instantly  liquefies,  but  the  water  turns  to 
ice,  the  solid  thus  becoming  a  liquid  and  the 
liquid  a  solid  by  the  exchange  of  heat.  A  cold 
knife  cuts  through  the  mass  of  frozen  mercury  as  a  hot  knife  would  ordinarily  through 
butter.  The  author,  on  one  occasion,  saw  Tyndall,  during  a  course  of  lectures  at 
the  Royal  Institution  at  London,  when  freezing  a  ladle  of  mercury  in  a  red-hot  cru- 
cible, add  some  ether  to  hasten  the  evaporation.  The  liquid  caught  fire,  but  the 
metal  was  drawn  out  from  the  glowing  crucible,  through  the  midst  of  the  flame, 
frozen  into  a  solid  mass. 


192  HEAT. 

6.  Spheroidal  State. — If  a  few  drops  of  water  be  put 
in  a  hot,  bright  spoon,  they  will  gather  in  a  globule,  which 
will  dart  to  and  fro  over  the  surface.  It  rests  on  a  cushion 
of  steam,  while  the  currents  of  air  drive  it  about.  If  the 
spoon  cool,  the  water  will  lose  its  spheroidal  form,  and 
coming  into  contact  with  the  metal,  burst  into  steam  with 
a  slight  explosion.* 

&    COMMUNICATION    OF    HEAT. 

Heat  tends  to  diffuse  itself  equally  among  surrounding 
bodies.f  There  are  three  modes  of  distribution. 

1.  Conduction  is  the  process  of  heating  by  the  passage  of 
heat  from  molecule  to  molecule.  Ex. :  Hold  one  end  of  a 
poker  in  the  fire,  and  the  other  end  soon  becomes  hot 
enough  to  burn  the  hand.  Of  the  ordinary  metals,  silver 
and  copper  are  the  best  conductors.  J  Wood  is  a  poor  con- 
ductor, especially  "  across  the  grain." 

Gases  are  the  poorest  conductors ;  hence  porous  bodies, 
as  wool,  fur,  snow,  charcoal,  etc.,  which  contain  large  quan- 
tities of  air,  are  excellent  non-conductors.  Refrigerators 
and  ice-houses  have  double  walls,  filled  between  with  char- 
coal, sawdust,  or  other  non-conducting  substances.  Air  is 
so  poor  a  conductor  that  persons  have  gone  into  ovens,  which 
were  so  hot  as  to  cook  meat,  which  they  carried  in  and  laid 
on  the  metal  shelves ;  yet,  so  long  as  they  did  not  them- 


*  Drops  of  water  spilled  on  a  hot  stove  illustrate  the  principle.— By  moistening 
the  finger,  we  can  touch  a  hot  flat-iron  with  impunity.  The  water  assumes  this  state, 
and  thus  protects  the  flesh  from  injury.— Furnace-men  can  dip  their  moistened  hands 
into  molten  iron.— 

t  If  we  touch  an  object  colder  than  we  are,  it  abstracts  heat  from  us,  and  we  say 
"  it  feels  cold  ; "  if  a  warmer  body,  it  imparts  heat  to  us,  and  we  say  "  it  feels  warm." 
Adjacent  objects  have,  however,  the  same  temperature,  though  flannel  sheets  feel 
warm,  and  linen  cold.  These  effects  depend  upon  the  relative  conducting  power  of 
different  substances.  Iron  feels  colder  than  feathers  because  it  robs  us  faster  of  our 
heat. 

\  Place  a  silver,  a  German-silver,  and  an  iron  spoon  in  a  dish  of  hot  water 
Notice  how  much  sooner  the  handle  of  the  silver  spoon  is  heated  than  the  others. 


COMMUNICATION-   OF   HEAT. 


193 


selves  touch  any  good  conductor,  they  experienced  little 
inconvenience. 

Liquids  are  also  poor  conductors.  Fia  181- 

Ex. :  Hold  the  upper  end  of  a  test- 
tube  of  water  in  the  flame  of  a  lamp. 
The  water  nearest  the  blaze  will  boil 
without  the  heat  being  felt  by  the 
hand. 

2.  Convection  is  the  process  of 
heating  by    circulation.      (1.)  CON- 
VECTION OF  LIQUIDS. — Place  a  little 
sawdust  in  a  flask  of  water,  and  ap- 
ply heat.     We  shall  soon  find  that  an 
ascending  and  a  descending  current 
are  established.     The  water  near  the 
lamp  becoming  heated,  expands  and 

rises.     The  cold  water  above  sinks  to  take  its  place. 

(2.)  OF  GASES. — By  testing  with  a  lighted  candle,  we 
shall  find  at  the  bottom  of  a  door  opening  into  cold  air,  a 
current  setting  inward,  and  at  the  top,  one  setting  outward. 
The  cold  air  in  a  room  flows  to  the  stove  along  the  floor,  is 
heated,  and  then  rises  to  the  ceiling.  Heating  by  hot-air 
furnaces  depends  upon  the  principle  that  warm  air  rises. 

3.  Radiation  is  the  transmission  of  heat-rays  in  straight 
lines.     The  heat  from  the  sun  comes  to  the  earth  in  this 
manner.     A  hot  stove  radiates  heat,     Rays  of  heat  do  not 
always  elevate  the  temperature  of    the  medium  through 
which  they  pass.     When  the  motion  of  the  ether-waves  is 
stopped,  the  effect  is  felt.*    Space  is  not  warmed  by  the 
sunbeam.     Meat  can  be  cooked  by  radiation,  while  the  air 
around  is  at  the  freezing-point.     A  rough,  unpolished  sur- 
face is  a  better  radiator  than  a  smooth,  bright  one.     Extent 
of  surface  increases  radiation.     Air,  vapor,  and  glass  allow 


*  The  Radiometer  is  a  curious  instrument  that,  for  a  time,  was  supposed  to 
exhibit  the  actual  mechanical  force  of  the  sunbeam.     It  consists  of  a  tiny  vane 


194 


luminous  rays  of  heat  to  pass  through  them  readily.  Thus 
the  heat  of  the  sunbeam  easily  penetrates  our  atmosphere, 
windows,  etc.  But  the  earth,  and  various  objects  on  its  sur- 
face, absorb  and  radiate  the  heat  back  again  as  obscure  heat 
in  long,  slow  wayes.  These  have  no  power  to  pass  the 
watery  vapor  in  the  air  or  through  glass.  The  moisture  of 
the  air  thus  acts  as  a  trap  to  catch  the  sunbeam  for  us.  If 
the  aqueous  vapor  were  removed,  the  earth  would  become 
uninhabitable.  On  the  desert  .of  Sahara,  where  "the  soil 
is  fire  and  the  wind  is  flame,"  the  dry  air  allows  the 
heat  to  escape  so  readily  that  ice  is  sometimes  formed  at 
night. 

Absorption  and  reflection  are  intimately  connected  with 
radiation.  A  good  absorber  is  also  a  good  radiator,  but  a 
good  reflector  can  be  neither.  Snow  is  a  good  reflector,  but 
a  poor  absorber  or  radiator.  Light  colors  absorb  solar  heat 

less  and  reflect  more  than  dark  colors.* 
FIG.  IBS.  White  is  generally  considered  the  best 

reflector,  and  black  the  best  absorber 

and  radiator. 


suspended  in  a  glass  globe  from  which  the  air  is  ex- 
hausted as  fully  as  possible,  when  the  globe  is  hermeti- 
cally sealed.  The  four  arms  of  the  vane  carry  each  a 
thin  pith  disk,  black  on  one  side  ind  white  on  the  other. 
When  daylight  falls  upon  it  the  vane  revolves  rapidly. 
The  motion  ceases  as  soon  as  the  light  is  removed. 
When  different  gases  are  admitted  into  the  globe,  the 
rate  of  rotation  varies.  It  is  now  thought  that  the  un- 
equal heating  of  the  black  and  white  surfaces  of  the 
disks  causes  a  reaction  of  the  gases,  and  the  force  of  this 
reaction  varies  in  accordance  with  the  rapidity  of  their 
molecular  vibrations.  See  p.  xii.  Fresh  Facts,  etc. 

*  Recent  experiments  show  that  with  artificial  heat 
the  molecular  condition  of  the  surface  varies  radiation  as 
well  as  reflection.  In  fact,  white  lead  is  as  good  a  radi- 
ator as  lampblack.— On  one  side  of  a  sheet  of  paper  paste 
letters  of  gold-leaf.  Spread  over  the  opposite  side  a  thin 
coating  of  scarlet  iodide  of  mercury— a  salt  which  turns 
yellow  on  the  application  of  heat.  (See  Chemistry,  p.  261.) 
Turn  the  scarlet  side  down.  Hold  over  the  paper  a  red- 
hot  iron.  The  gold-leaf  will  reflect  the  heat,  but  the  paper 
spaces  between  the  letters  will  absorb  it,  and  on  turning 
the  paper  over,  the  gilt  letters  will  be  found  traced  in 
scarlet  on  a  yellow  background.  See  p.  xii  Fresh  Facts. 


THE    STEAM-ENGINE. 


195 


FIG.  183. 


4.    THE  STEAM-ENGINE. 

When  steam  rises  from  water  at  a  temperature  of  212°  it 
has  an  elastic  force  of  15  Ibs.  per  square  inch.  If  the  steam 
be  confined  and  the  temperature  raised,  the  elastic  force  will 
be  rapidly  increased. 

1.  The  Steam-engine  is  a  ma- 
chine for  using  the  elastic  force  of 
steam  as  a  motive  power.  There  are 
two  classes,  high-pressure  and  low- 
pressure.  In  the  former,  the  steam, 
after  it  has  done  its  work,  is  forced 
out  into  the  air;  in  the  latter,  it  is 
condensed  in  a  separate  chamber  by 
a  spray  of  cold  water.  *  The  figure 
represents  the  piston  and  connecting 
pipes  of  an  engine.  The  steam  from 
the  boiler  passes  through  the  pipe 
into  the  steam-chest,  as  indicated  by 
the  arrow.  The  sliding-valve  worked 
by  the  rod  h  lets  the  steam  into  the  cylinder,  alternately 
above  and  below  the  piston,  which  is  thus  made  to  play 
up  and  down  by  the  expansive  force. 


FIG.  184. 


2.  The  Governor  is  an  apparatus 
for  regulating  the  supply  of  steam.    AB 
is  the  axis  around  which  the  heavy  balls 
E  and  D  revolve.     When  the  machine 
is  going  too  fast  the  balls  fly  out  and 
shut  off  a  portion  of  the  steam ;  when 
too  slowly,  they  fall  back,  and,  opening 
the  valve,  let  on  the  steam  again. 

3.  A  High-pressure  Engine  is 


*  As  the  steam  is  condensed  in  the  low-pressure  engine,  a  vacuum  is  formed 
behind  the  piston;  while  the  piston  of  the  high-pressure  engine  acts  against  the 
pressure  of  the  air.  The  elastic  force  of  the  steam  must  be  15  Ibs.  per  square  inch 
greater  in  the  latter  case. 


METEOROLOGY.  197 

shown  in  Fig.  185.  A  represents  the  cylinder,  B  the  steam- 
chest,  0  the  throttle-valve  in  the  pipe  through  which  steam 
is  admitted  from  the  boiler,  D  the  governor,  E  the  band- 
wheel  by  which  the  governor  is  driven,  F  the  pump,  G  the 
crank,  I  the  conductor  attached  to  a  the  cross-head,  H  the 
eccentric  rod  (h  in  Fig.  183)  which  works  the  sliding-valve 
in  the  steam-chest,  K  the  governor-valve,  S  the  shaft  by 
which  the  power  is  conveyed  to  the  machinery.  The  cross- 
head,  «,  slides  to  and  fro  in  a  groove,  and  is  fastened  to  the 
rod  which  works  the  piston  in  the  cylinder  A.  The  expan- 
sive force  of  the  steam  is  thus  communicated  to  «,  thence 
to  I,  by  which  the  crank  is  turned.  The  heavy  fly-wheel 
renders  the  motion  uniform  (p.  78). 

5.    METEOROLOGY. 

1.  General  Principles. — (1.)  The  air  always  contains 
moisture.     The  amount  it  can  receive  depends  upon  the 
temperature ;  warm  air  absorbing  more,  and  cold  air  less. 
At  75°,  a  cubic  yard  of  air  can  hold  over  half  an  ounce  of 
water;  a  reduction  of  27°  will  cause  half  that  quantity  to 
be  deposited.     When  the  air  at  any  temperature  contains  all 
the  vapor  it  can  hold,  it  is  said  to  be  saturated  ;  any  fall  of 
temperature  will  then  condense  a  part  of  the  vapor. 

(2.)  When  air  expands  against  pressure  (i.  e.,  doing  work 
in  the  expansion)  its  sensible  heat  becomes  latent,  and 
there  is  a  fall  of  its  temperature.  The  warm  air  from  the 
earth  ascending  into  the  upper  regions,  is  thus  rarefied  and 
cooled.  Its  vapor  is  then  condensed  into  clouds,  and  often 
falls  as  rain.  Owing  to  this  expansion  of  the  atmosphere 
and  the  greater  radiation  of  heat  in  the  dry  air  of  the  upper 
regions,  there  is  a  gradual  diminution  of  the  temperature 
as  the  altitude  increases,  the  mean  rate  in  the  north  tem- 
perate zone  being  about  1°  for  300  feet. 

2.  Dew.*— The  grass  at  night,  becoming  cooled  by  radi- 

*  Dew  was  anciently  thought  to  possess  wonderful  properties.  Baths  in  this 
precious  liquid  were  said  greatly  to  conduce  to  beauty.  It  was  collected  for  this 
purpose,  and  for  the  use  of  the  alchemists  in  their  weird  experiments,  by  spreading 


198  HEAT. 

ation,  condenses  upon  its  surface  the  yapor  of  the  air.     Dew 

will  gather  most  freely  upon  the  best  radiators,  as  they  will 
the  soonest  become  cool.  Thus  grass,  leaves,  etc.,  which 
need  the  most,  get  the  most.  It  will  not  form  on  windy 
nights,  because  the  air  is  constantly  changing  and  does  not 
become  cool  enough  to  deposit  its  moisture.  In  tropical 
regions  the  nocturnal  radiation  is  often  so  great  as  to  form 
ice.  In  Bengal,  water  is  exposed  for  this  purpose  in  shallow 
earthen  dishes  resting  on  rice  straw.  The  most  dew  collects 
on  a  clear,  cloudless  night.  *  In  Chili,  Arabia,  etc.,  by  its 
abundance,  it  supplies  the  place  of  rain.  When  the  tem- 
perature of  plants  falls  below  32°,  the  vapor  is  frozen  upon 
them  directly,  and  is  called  white  or  hoar-frost. 

3.  Fogs  are  formed  when  the  temperature  of  the  air  falls 
below  the  dew-point,  i.  e.,  the  temperature  at  which  dew  is 
deposited.     They  are  characteristic  of  low  lands,  rivers,  etc., 
where  the  air  is  saturated  with  moisture. 

4.  Clouds  differ  from  fogs  only  in  their  elevation  in  the 
atmosphere.     They  are  formed  when  a  warm,  humid  wind 
penetrates  a  cold  air,  or  a  cold  wind  a  warm,  humid  air. 
Clouds  are  constantly  falling  by  their  weight,  but  as  they 
melt  in  the  warm  air  below,  by  condensation  they  increase 
above. 

The  nimbus  cloud  is  a  dark-colored  cloud  from  which  rain 
is  falling. 

The  stratus  cloud  is  composed  of  broad,  widely-extended 
cloud-belts,  sometimes  spread  over  the  whole  sky.  It  is  the 
lowest  cloud,  and  often  rests  on  the  earth,  where  it  forms  a 
fog.  It  is  the  night-cloud. 

The  cumulus  cloud  is  made  up  of  large  cloud-masses  look- 
ing like  snow-capped  mountains  piled  up  along  the  horizon. 
It  forms  the  summits  of  pillars  of  vapor,  which,  streaming 
up  from  the  earth,  are  condensed  in  the  upper  air.  It  is 

fleeces  of  wool  upon  the  ground.  Laurens,  a  philosopher  of  the  middle  ages,  claimed 
that  dew  is  ethereal,  BO  that  if  we  should  fill  a  lark's  egg  with  it  and  lay  it  out  in  the 
8tin.  immediately  on  the  rising  of  that  luminary,  the  egg  would  fly  off  into  the  air » 


MBTBOBOL08Y. 


199 


the  day-cloud.     When  of  small  size  and  seen  near  mid-day, 
it  is  a  sign  of  fair  weather. 

The  cirrus  (curl)  •  cloud  consists  of  light,  fleecy  clouds 
floating  high  in  air.  It  is  composed  of  spiculss  of  ice  or 
flakes  of  snow. 

PIG.  186. 


Different  kinds  of  clouds— 1  bird  indicates  the  nimbus,  2  birds  the  stratus, 
8  birds  the  cumulus,  and  4  birds  the  cirrus  cloud. 

The  cirro-cumulus  is  formed  by  small  rounded  portions 
of  cirrus  cloud,  having  a  clear  sky  between.  Sailors  call 
this  a  "mackerel  sky."  It  accompanies  warm,  dry  weather. 

The  cirro-stratus  is  produced  when  the  cirrus  cloud 
spreads  into  long,  slender  strata.  It  forebodes  storms. 

The  cumulo-stratus  presents  the  peculiar  form  called 
"  thunder-head."  It  is  caused  by  a  blending  of  the  cumulus 
with  the  stratus,  and  is  a  precursor  of  thunder-storms. 

5.  Bain  is  vapor  condensed  by  the  sudden  cooling  of  the 
air  in  the  upper  regions.  At  a  low  temperature  the  vapor  is 
frozen  directly  into  snow.  This  may  melt  before  it  reaches 


200 


the  earth,  and  fall  as  rain.  A  sudden  draught  oi  cold  air 
into  a  heated  ball-room  has  produced  a  miniature  snow- 
storm. The  wonderful  variety  and  beauty  of  snow-crystals 
are  illustrated  in  the  figure. 

PIG.  187. 


6.  Winds  are  produced  by  variations  in  the  temperature 
of  the  air.  The  atmosphere  at  some  point  is  expanded, 
rises,  and  colder  air  flows  in  to  supply  its  place.  This  pro- 
duces currents.  The  land  and  sea  breezes  of  tropical  islands 
are  caused  by  the  unequal  specific  heat  of  land  and  water. 
During  the  day  the  land  becomes  more  highly  heated  than 
the  water,  and  hence  toward  evening  a  sea-breeze  sets  in 
from  the  ocean.  At  night  the  land  cools  faster  than  the 
water,  and  so  toward  morning  a  land-breeze  sets  out  from 
the  land. 

Trade-winds  are  so  named  because  by  their  regularity  they 
favor  commerce.  A  vessel  on  the  Atlantic  Ocean  will  some- 
times, without  shifting  a  sail,  set  steadily  before  this  wind 
from  Cape  Yerde  to  the  American  coast.  The  air  about 
the  equator  is  highly  heated,  and,  rising  to  the  upper 


METEOROLOGY.  201 

regions,  flows  off  north,  and  south.  The  cold  air  near  the 
poles  sets  toward  the  equator  to  fill  its  place.  If  the  earth 
were  at  rest  this  would  make  an  upper  warm  current  flowing 
from  the  equator,  and  a  lower  cold  current  flowing  toward  it. 
As  the  earth  is  revolving  on  its  axis  from  west  to  east,  the 
under  current  starting  from  the  poles  is  constantly  coming 
to  a  part  moving  faster  than  itself.  It  therefore  lags  behind. 
When  it  reaches  the  north  equatorial  regions  it  lags  so  much 
that  it  becomes  a  current  from  the  northeast,  and  in  the 
south  equatorial  regions  a  current  from  the  southeast. 

7.  Ocean  Currents  are  produced  in  a  similar  manner. 
The  water  heated  by  the  vertical  sun  of  the  tropics  rises  and 
flows  toward  the  poles.     The  Gulf  Stream,  which  issues  from 
the  Gulf  of  Mexico,  carries  the  heat  of  the  Caribbean  Sea 
across  the  Northern  Atlantic  to  the  shores  of  Scotland  and 
Norway.     This  tropical  river  flowing  steadily  through  the 
cold  water  of  the  ocean,  rescues  England  from  the  snows  of 
Labrador.     Should  it  by  any  chance  break  through  the 
Isthmus  of  Panama,  Great  Britain  would  be  condemned  to 
arctic  glaciers.     (See  p.  xii.     Fresh  Facts  and  Theories.) 

8.  Adaptations  of  "Water. — The  great  specific  heat  of 
water  exercises  a  marked  influence  on  climate.      Warm 
winds  sweeping  northward  meet  the  colder  air  of  the  tem- 
perate regions  and  deposit  their  moisture.     The  latent  heat 
which  the  vapor  absorbed  in  the  sunny  South  is  set  free,  to 
temper  the  severity  of  our  snowy  climate.     Thus,  aerial  and 
oceanic  currents  constitute  a  water  circulation  which  is  a 
natural  steam-apparatus  on  the  grandest  scale,  having  a 
boiler  at  the  equator  and  steam-pipes  running  over  the 
globe.     Water  also  tends  to  prevent  sudden  changes  of 
weather.     In  the  summer  it  absorbs  vast  quantities  of  heat, 
which  it  gives  off  in  the  fall  to  moderate  the  approach  of 
winter.     In  the  spring  the  melting  ice  and  snow  drink  in 
the  warmth  of  the  sunbeam,  which  else  might  prematurely 
coax  forth  the  tender  buds.     Since  so  much  heat  is  required 
fco  melt  the  ice  and  snow,  they  dissolve  very  slowly,  and  thus 


HEA.T. 

ward  off  the  disastrous  floods  which  would  follow,  if  they 
passed  quickly  into  the  liquid  state. 

Water  contains  air,  which  is  necessary  for  the  support  of 
fish.  Just  here  occurs  one  of  those  happy  coincidences 
which  frequently  startle  the  reverent  searcher  in  Nature. 
Were  water  deprived  of  air,  it  would  be  liable  to  explode  at 
any  moment  when  it  happened  to  be  heated  above  212°. 
Every  stove-boiler  would  need  a  thermometer.  A  tea-kettle 
would  require  as  careful  watching  as  a  steam-engine,  and 
our  kitchens  would  witness  frequent  and  perhaps  disastrous 
explosions. 

Water,  like  other  liquids,  expands  with  heat,  and  con- 
tracts, on  cooling,  down  to  39°  F.*  Then  it  slowly  expands 
until  it  reaches  32°  F.,  when  it  freezes,  f  The  bursting  of 
water-pipes  and  pails  is  a  familiar  example  of  this  excep- 
tion. Under  the  operation  of  the  general  law,  the  water  at 
the  surface  radiating  its  heat  and  becoming  chilled,  would 
contract  and  fall  to  the  bottom,  while  the  warm  water  below 
would  rise  to  the  top.  This  process  would  continue  until 
the  freezing-point  was  reached,  when  the  whole  mass  would 
solidify  into  ice.  Our  lakes  and  rivers  would  freeze  solid 
every  winter.  This  would  be  fatal  to  the  fish.  In  the 
spring,  the  ice  would  not,  as  now,  buoyant  and  light,  float 
and  melt  in  the  direct  sunbeam,  but,  lying  at  the  bottom, 
would  be  protected  by  the  non-conducting  water  above. 
The  longest  summer  would  not  be  sufficient  to  thaw  the 
deeper  bodies  of  water.  An  exception  prevents  these  calam- 


*  Since  ice  when  it  melts  contracts,  pressure  aids  in  liquefaction  and  so  lowers 
the  melting-point  (p.  190).  A  glacier  descending  a  mountain  melts  at  every  place 
where  there  is  pressure  and  freezes  again  when  the  obstacle  is  passed.  Bits  of  ice 
when  squeezed  freeze  together.  This  property  is  called  regelation.  In  the  winter, 
snow  is  packed  into  ice  by  loaded  wagous. 

t  Pit  a  small  flask  with  a  cork,  through  which  passes  an  upright  glass  tube.  Fill 
with  colored  water  Apply  heat  to  the  flask  until  the  liquid  runs  over  the  top  of  the 
tube.  This  shows  the  expansion  by  heat.  Now  apply  a  freezing  mixture  to 
the  flask,  and  at  first  the  liquid  in  the  tube  falls,  but  soon  begins  to  rise.  When  it 
runs  over  as  before,  apply  heat  and  it  shrinks  back  again.  Thus  cold  will  expand 
and  heat  contract  it.  When  water  is  at  its  maximum  density  (about  39°)  expansion 
&pts  in  alike,  whether  you  heat  or  cool  it. 


METEOBOLOGY.  203 

itous  consequences.*  The  cold  water  expanding  and  rising 
to  the  top  protects  the  warm  water  beneath,  while  ice  itself, 
being  a  non-conductor,  preserves  the  temperature  during 
the  winter. 

Water,  f  in  freezing,  has  a  tendency  to  free  itself  from 
impurities.  Thus,  melting  ice  furnishes  a  means  of  obtain- 
ing fresh  water  in  Arctic  regions.  If  a  barrel  of  vinegar 
freeze,  we  shall  find  the  acid  collected  in  a  little  mass  at 
the  centre  of  the  ice. 

When  the  dew  gathers  at  night  sufficiently  to  form  a 
covering  upon  the  plants,  being  a  non-conductor,  it  stops 
farther  radiation  of  heat.  Thus,  by  a  nice  provision,  the 
effect  of  radiation  checks  the  radiation  itself,  as  soon  as  the 
wants  of  the  thirsty  vegetation  are  supplied. 

PRACTICAL  QUESTIONS.— 1.  Why  will  one's  hand,  on  a  frosty  morning,  freeze 
to  a  metallic  door-knob  soonar  than  to  one  of  porcelain  ?  2.  Why  does  a  piece  of 
bread  toasting  curl  up  on  the  side  exposed  to  the  fire  ?  3.  Why  do  double  windows 
protect  from  the  cold  ?  4.  Why  do  furnace-men  wear  flannel  shirts  in  summer  to 
keep  cool,  and  in  winter  to  keep  warm  ?  5.  Why  do  we  blow  our  hands  to  make 
them  warm,  and  our  soup  to  make  it  cool  ?  6.  Why  does  snow  protect  the  grass  in 
winter  ?  7.  Why  does  water  "boil  away  "  more  rapidly  on  some  days  than  on  others  ? 
8.  What  causes  the  crackling  sound  of  a  stove  when  a  fire  is  lighted  ?  9.  Why  is  the 
tone  of  a  piano  higher  in  a  cold  room  than  in  a  warm  one  ?  10.  Ought  an  inkstand 
to  have  a  large  or  a  small  mouth  ?  11.  Why  is  there  a  space  left  between  the  ends 
of  the  rails  on  a  railroad  track  ?  s2.  Why  is  a  person  liable  to  take  cold  when  his 
clothes  are  damp  ?  13.  What  i.3  the  theory  of  corn-popping  ?  14.  Could  vacuum- 
pans  be  employed  in  cooking  ?  15.  Why  does  the  air  feel  so  chilly  in  the  spring, 
when  snow  and  ice  are  melting  ?  16.  Why,  in  freezing  ice-cream,  do  we  put  the  ice 
in  a  wooden  vessel  and  the  cream  in  a  tin  one  ?  17.  Why  does  the  temperature 
generally  moderate  when  snow  falls  ?  18.  What  causes  the  singing  of  a  tea-kettle  ? 
Ans.  The  escaping  steam  is  thrown  into  vibration  by  the  shape  of  the  spout.  19. 
Why  does  sprinkling  a  floor  with  water  cool  the  air  ?  20.  How  low  a  degree  of  tem- 


*  Certain  metals— iron,  bismuth,  etc.— are  also  exceptions  to  the  general  law. 
This  fact  adapts  them  for  castings.  Who  shall  say  these  are  not  all  thoughtful  pro- 
visions for  our  wants  ? 

t  Water  distills  from  the  ocean  and  land  as  vapor,  at  one  time  cooling  and 
refreshing  the  air,  at  another  moderating  its  wintry  rigor.  It  condenses  into  clouds, 
which  shield  the  earth  from  the  direct  rays  of  the  sun,  and  protect  against  excessive 
radiation.  It  falls  as  rain,  cleansing  the  air  and  quickening  vegetation  with  renewed 
life.  It  descends  as  snow,  and,  like  a  coverlet,  wraps  the  grass  and  tender  buds  in 
its  protecting  embrace.  It  bubbles  up  in  springs,  invigorating  us  with  cooling, 
healing  draughts  in  the  sickly  heat  of  summer.  It  purifies  our  system,  dissolves  our 
food,  and  keeps  our  joints  supple.  It  flows  to  the  ocean,  fertilizing  the  soil,  and 
floating  the  products  of  industry  and  toil  10  the  markets  of  the  world.  (See 
tetry,  pp.  56-63J 


204  HEAT. 

perature  can  be  marked  by  a  mercurial  thermometer  ?  21.  If  the  temperature  is  70°F., 
what  is  it  C.?  22.  Will  dew  form  on  an  iron  bridge  ?  On  a  plank  walk  ?  23.  Why 
will  not  corn  pop  when  very  dry?  24.  When  the  interior  of  the  earth  is  so  hot,  why 
do  we  get  the  coldest  water  from  a  deep  well?  25.  Ought  the  bottom  of  a  tea-kettle 
to  be  polished  ?  26.  Which  boils  the  sooner,  milk  or  water?  27.  Is  it  economy  to 
keep  our  stoves  highly  polished?  28.  If  a  thermometer  be  held  in  a  running  stream, 
will  it  indicate  the  same  temperature  that  it  would  in  a  pailful  of  the  same  water  ? 
29.  Which  makes  the  better  "•  holder,"  woollen  or  cotton  ?  30.  Which  will  give  out 
the  more  heat,  a  plain  stove  or  one  with  ornamental  designs  ?  31.  Does  dew  fall  ? 
32.  What  causes  the  "  sweating"  of  a  pitcher  ?  33.  Why  is  evaporation  hastened  in 
a  vacuum  ?  34.  Does  stirring  the  ground  around  plants  aid  in  the  deposition  of 
dew  ?  35.  Why  does  the  snow  at  the  foot  of  a  tree  melt  sooner  than  that  in  the 
open  field  ?  36.  Why  is  the  opening  in  a  chimney  made  to  decrease  in  size  from 
bottom  to  top  ?  37.  Will  tea  keep  hot  longer  in  a  bright  or  a  dull  teapot  ?  38. 
What  causes  the  snapping  of  wood  when  laid  on  the  fire  ?  Ans.  The  expansion  oi 
the  air  in  the  cells  cf  the  wood.  89.  Why  is  one's  breath  visible  on  a  cold  day? 
40.  What  gives  the  blue  color  to  air  ?  Ans.  The  opaque  particles  floating  in  it  reflect 
the  blue  light  of  the  sunbeam.  41.  Why  is  light-colored  clothing  cooler  in  summer 
and  warmer  in  winter  than  dark?  42.  How  does  the  heat  at  two  feet  from  the  fire 
compare  with  that  at  four  feet  ?  43.  Why  does  the  frost  remain  later  in  the  morning 
upon  some  objects  than  upon  others?  44.  Is  it  economy  to  use  green  wood ?  46. 
Why  will  a  piece  of  metal  dropped  into  a  glass  or  porcelain  dish  of  boiling  water 
increase  the  ebullition  ?  47.  Which  can  be  ignited  the  more  quickly  with  a  burning- 
glasp,  black  or  white  paper  ?  48.  Why  does  the  air  feel  colder  on  a  windy  day  ?  49. 
In  what  did  the  miracle  of  Gideon's  fleece  consist?  50.  Could  a  burning-lens  be 
made  of  ice  ?  51.  Why  is  an  iceberg  frequently  enveloped  by  a  fog.  52.  Would  dew 
gather  more  freely  on  a  rusty  stove  than  on  a  bright  kettle  ?  53.  Why  is  a  clear 
night  generally  colder  than  a  cloudy  one  at  the  same  season  ?  54.  Why  is  no  dew 
formed  on  cloudy  nights?  55.  Why  will  "fanning"  cool  the  face?  56.  How  are 
safes  made  fire-proof?  57.  Why  can  you  heat  water  quicker  in  a  tin  than  a  china 
cup  ?  58.  Why  will  a  woollen  blanket  keep  ice  from  melting  ?  59.  Does  dew  form 
under  trees?  60.  What  is  the  principle  of  heating  by  steam?  62.  What  is  the 
cause  of  "  cloud-capped  "  mountains  ?  63.  Show  how  the  glass  in  a  hot-house  acts 
as  a  trap  to  catch  the  sunbeam.  64.  Does  the  heat  of  the  sun  come  in  through  our 
windows?  65.  Does  the  heat  of  our  stoves  pass  out  in  the  same  way?  66.  Is  a 
heavy  dew  a  sign  of  rain  ?  Ans.  Yes  ;  because  it  shows  that  the  moisture  of  the  air 
is  easily  condensed.  67.  Is  a  dusty  boot  hotter  to  the  foot  than  a  polished  one  ?  68. 
The  top  of  a  mountain  is  nearer  the  sun,  why  is  it  not  warmer  ?  69.  What  it»  hoar- 
frost ?  Ans.  Frozen  dew.  70.  Why  will  a  plight  covering  protect  plants  from  frost  ? 
Ans.  Because  it  prevents  radiation.  71.  Why  is  there  no  frost  on  cloudy  nights? 
Ans.  The  clouds  act  like  a  blanket,  to  prevent  radiation  and  keep  the  earth  warm. 

72.  Can  we  find  frost  on  the  windows  and  on  the  stone-flagging  the  same  morning  ? 

73.  Why  will  not  snow  "  pack"  into  balls  except  in  mild  weather  ?    74.  Why  is  the 
sheet  of  zinc  under  a  stove  so  apt  to  become  puckered?    75.  Why  does  a  mist 
gather  in  the  receiver  of  the  air-pump  as  the  air  becomes  rarefied  ?    76.  Why  are 
the  tops  of  high  mountains  in  the  tropics  covered  with  perpetual  snow  ? 


6CMMARY.  806 


SUMMARY. 

is  produced  by  longer  and  less  refrangible  waves  and  slower 
vibrations  of  ether  than  those  which  cause  light.  Both  luminous  and 
obscure  heat  may  be  radiated,  reflected,  refracted,  absorbed,  focused, 
and  polarized  in  precisely  the  same  manner  as  the  light-force.  In 
fact,  light  is  only  visible  radiant  heat.  If  we  elevate  the  temperature 
of  a  body  sufficiently,  we  can  change  heat-rays  into  light-rays.  A 
body  which  allows  the  radiant  heat  to  pass  through  it  easily  is  styled 
diathermanous  ;  rock-salt  is  such  a  body,  being  to  heat-rays  what  glass 
is  to  light-rays.  The  sun  is  the  principal  source  of  heat.  But  heat 
can  be  obtained  by  chemical  and  mechanical  means.  In  burning  coal 
we  secure  it  by  the  former  method.  Mechanical  force  may  be  changed 
directly  into  heat,  as  in  striking  fire  with  flint  and  steel,  and  in  ham- 
mering a  bullet  on  an  anvil  until  it  is  hot.  According  to  Joule's  law, 
772  feet  fall  of  a  weight  corresponds  to  1°  of  temperature  in  the  same 
amount  of  water.  (Recent  investigations  by  Joule  and  others  indicate 
that  780  feet  is  nearer  the  exact  truth.) 

Among  the  physical  effects  of  heat  are  a  change  of  temperature, 
expansion,  liquefaction,  vaporization,  and  evaporation.  The  heat- 
force  increases  the  ms  viva  of  the  molecules,  thus  elevating  the  tem- 
perature; and  the  increased  vibration  of  the  molecules  causes  an 
expansion  of  the  body.  The  latter  is  so  uniform  that  it  is  used  to 
indicate  changes  of  temperature,  as  in  the  thermometer.  The  expan- 
sion of  the  metals  by  heat  is  turned  to  account  in  many  art  processes. 
The  walls  of  a  gallery  in  the  Conservatoire  des  Arts  et  Metiers  in 
Paris,  had  begun  to  bulge.  To  remedy  this,  iron  rods  were  passed 
across  the  building  and  screwed  into  plates  on  the  outside  of  the  walls. 
By  heating  the  bars,  they  were  expanded,  when  they  were  screwed 
up  tightly.  Being  then  allowed  to  cool  they  contracted,  thus  drawing 
the  walls  back  toward  a  perpendicular. 

Heat  is  the  great  antagonist  of  cohesion.  The  liquid  and  gaseous 
states  of  bodies  depend  on  its  relative  presence  or  absence  (absolute 
cold  is  as  yet  only  a  theoretical  condition,  all  bodies  with  which  we 
are  familiar  containing  heat).  When  the  heat  force  nearly  balances 
the  cohesive,  the  body  breaks  down  into  a  liquid,  and  when  the  repel 
lent  fairly  triumphs,  the  particles  fly  off  as  a  gas.  Immediately  before 
and  after  each  of  these  marked  changes,  viz. :  of  a  solid  to  a  liquid 
and  of  a  liquid  to  a  gas,  the  thermometer  indicates  the  same  temper- 
ature. Thus  water  from  melting  ice  affects  the  thermometer  just  as 
the  ice  does,  and  steam  is  no  hotter  than  the  boiling  water  The  heat 


206  HEAT. 

which,  in  these  processes,  becomes  hidden  from  the  thermometer  is 
called  latent,  though  we  now  know  that,  being  occupied  in  doing 
internal  work,  it  has  merely  taken  the  static  form,  and  can  be  readily 
turned  again  into  the  dynamic.  The  so-called  latent  heat  of  water  is 
only  the  potential  heat-energy  of  the  separated  molecules,  which  will 
reappear  the  instant  the  molecules  collapse  and  come  once  more  within 
the  grasp  of  cohesion.  On  this  principle  is  based  the  method  of  heat- 
ing by  steam.  Evaporation  is  a  slow  vaporization  that  takes  place  at 
all  temperatures,  but  may  be  greatly  increased  by  a  diminution  of 
pressure,  as  in  a  vacuum.  It  is  a  cooling  process,  and  is  practically 
applied  to  the  manufacture  of  ice. 

By  the  subtraction  of  heat,  i.  e.,  by  cold,  and  by  the  addition  of 
pressure,  which  antagonizes  the  repellent  heat  force,  gases  may  be 
liquefied  and  even  congealed,  the  transparent  carbonic-acid  gas  thus 
becoming  a  snowy  solid.  What  were  formerly  called  the  "permanent 
gases"  (oxygen,  hydrogen,  etc.),  have  recently  (Dec.,  1877)  been  lique- 
fied by  means  of  the  cold  produced  by  their  rarefaction  (see  p.  197) 
when  they  were  suddenly  released  from  a  pressure  of  two  or  three 
hundred  atmospheres. 

Heat  is  conducted  from  molecule  to  molecule  of  a  body,  radiated  in 
straight  lines  through  air  (or  space),  and  circulated  by  the  transfer- 
ence of  heated  masses  through  a  change  of  specific  gravity  due  to 
expansion.  The  first  method  is  characteristic  of  solids,  and  the  third 
of  liquids  and  gases.  The  elastic  force  of  steam  increases  when  it  is 
confined  and  a  higher  temperature  is  reached.  The  steam-engine 
utilizes  this  principle.  There  are  two  forms  of  this  machine,  the 
high-pressure  and  the  low-pressure,  according  as  the  waste  steam  is 
ejected  into  the  air  or  condensed  in  a  separate  chamber.  The  phe- 
nomena of  dew,  rain,  etc.  depend  upon  the  fact  that  a  change  from  a 
higher  to  a  lower  temperature  causes  the  air  to  deposit  its  moisture. 


HISTORICAL    SKETCH. 

Democritus,  the  originator  of  the  Atomic  Theory,  held  that  heat 
consists  of  minute  spherical  particles  radiated  rapidly  enough  to  pen- 
etrate every  substance.  Until  very  recently  heat  and  light  were  thus 
reckoned  among  the  Imponderables,  i.  e.,  matter  which  has  no  weight. 
Still,  even  in  early  times,  some  philosophers  caught  glimpses  of  a  true 
theory.  Aristotle  considered  heat  more  a  condition  than  a  substance. 
Bacon,  in  his  Novum  Organum,  wrote  :  "  Heat  is  a  motion  of  expan- 
sion ; "  and  elsewhere,  "  Essential  heat  is  motion  and  nothing  else. " 
Locke,  half  a  century  later,  said  :  "  Heat  is  a  very  brisk  agitation  of 


HISTORICAL    SKETCH.  20? 

the  insensible  parts  of  an  object,  which,  produces  in  us  the  sensation 
from  whence  we  denominate  the  object  hot,  so  that  what  in  our  sen- 
sation is  heat,  in  the  object  is  motion." 

The  material  view,  however,  held  its  ground.  At  the  beginning  of 
the  18th  century,  Stahl  elaborated  a  theory  that  a  buoyant  substance 
called  phlogiston  is  the  principle  of  heat,  and  that  when  a  body  burns, 
its  phlogiston  escapes  as  fire.  In  1760,  Dr.  Black  investigated  and  made 
known  the  principles  of  what  he  termed  latent  heat,  i.  e. ,  heat  which 
becomes  hidden  when  ice  is  turned  into  water  or  water  into  steam. 
Priestley  discovered,  in  1774,  and  Lavoisier  afterward  developed,  the 
modern  view  of  combustion.  But  the  latter  philosopher  then  advanced 
the  theory  that  heat  (caloric)  is  an  actual  substance,  which  passes  freely 
from  one  body  to  another  and  combines  at  pleasure.  This  caloric  the- 
ory still  holds  its  place  in  our  older  philosophies.  Toward  the  close  of 
the  18th  century,  Benjamin  Thompson,  better  known  as  Count  Rum- 
ford,  a  native  of  Woburn,  Mass.,  but  in  the  employ  of  the  Elector  of 
Bavaria,  proved  the  convertibility  of  force.  "  He  first  took  the  sub- 
ject," as  Prof.  Youmans  well  remarks,  "out  of  the  domain  of  meta- 
physics, where  it  had  been  speculated  upon  since  the  time  of  Aristotle, 
and  placed  it  on  the  true  basis  of  physical  experiment." 

Rumford  was  led  to  investigate  the  nature  of  heat  from  noticing 
in  the  workshops  at  Munich,  how  hot  the  cannon  became  while  boring. 
There  seemed  to  be  no  limit  to  the  amount  of  heat  which  could  be 
produced,  yet  the  cannon,  the  borer  and  the  chips  lost  nothing,  so  far 
as  he  could  detect.  If  heat  were  a  fluid,  as  the  caloric  theory  asserted, 
then  there  should  be  an  end  to  the  process,  sooner  or  later.  Rurnford 
now  began  to  get  gleams  of  the  truth  of  the  vibratory  theory.  Taking 
a  large  piece  of  brass  with  a  hollow  at  one  end,  he  fitted  to  it  a  blunt 
steel  borer,  which  pressed  down  upon  the  metal  with  a  weight  of 
10,000  Ibs.  This  apparatus  he  placed  in  a  box  holding  about  18f  Ibs. 
of  water.  The  brass  was  then  made  to  revolve  by  horse-power  at  the 
rate  of  32  times  per  minute.  In  the  beginning  the  water  in  the  box 
was  at  60°  F.,  but  in  two  hours  and  a  half  it  actually  boiled.  "  It 
would  be  difficult,"  wrote  Rumford,  "to  describe  the  surprise  and 
astonishment  of  the  bystanders  to  see  so  large  a  quantity  of  water 
heated  and  actually  made  to  boil  without  any  fire."  He  naively  adds 
that  he  was  himself  as  delighted  as  a  child,  and  forgot  all  the  dignity 
becoming  a  philosopher.  By  this  experiment  he  had  proved  that  mo- 
tion can  be  turned  into  heat.  His  experiments,  however,  conclusive  as 
they  now  seem  to  be,  were,  at  the  time,  the  subject  of  ridicule.  Sir 
Humphrey  Davy  next  began  to  see  the  truth,  and,  in  1799,  melted  two 
pieces  of  ice  by  friction  in  a  vacuum. 

Soon  the  scientific  world  seemed  to  be  ripe  for  this  discovery,  and 
it  appears  to  have  sprung  up  spontaneously  in  men's  thoughts  every- 


208  HEAT. 

where.  Mayer,  a  physician  of  Germany,  and  Grove,  of  England, 
proved  the  mutual  relation  of  the  forces,  the  latter  first  using  the  term 
"Correlation  of  Forces,"  since  changed  to  Conservation  of  Energy. 
Joule  discovered  the  law  of  the  "Mechanical  Equivalent  of  Heat," 
about  1843.  In  his  famous  experiments  he  used  pound- weights  made 
to  fall  through  a  measured  distance.  Cords  were  attached  to  them,  so 
that,  as  they  fell,  they  turned  a  paddle-wheel  placed  in  a  box  of  water. 
Other  liquids  were  used  instead  of  the  water.  The  rise  of  temperature 
in  the  liquids  was  carefully  marked.  The  loss  by  friction  in  the  appa- 
ratus was  estimated,  and  so,  at  last,  the  dynamical  theory  of  heat  was 
fully  demonstrated.  Names  of  philosophers  well  known  to  us,  such  as 
Henry,  Helmholtz,  Faraday,  Thomson,  Maxwell,  Leconte,  Youmans, 
Stewart,  and  Tyndall,  are  associated  with  the  final  establishment  of 
this  theory. 

Consult,  on  this  interesting  subject,  Tait's  "Recent  Advances  in 
Physical  Science ;"  Stewart's  " Treatise  on  Heat;"  Tyndall's  "Heat  a 
Mode  of  Motion ; "  Maxwell's  "Theory  of  Heat ; "  Thurston's  "  History 
of  the  Growth  of  the  Steam  Engine  ; "  Buckley's  "  Short  History 
of  Natural  Science  ; "  Smiles's  "  Lives  of  Boulton  and  Watt ; "  You- 
mans's  "Correlation  of  the  Physical  Forces  ;"  "Read  and  the  Steam- 
Engine;"  American  Cyclopaedia,  Art.  "Steam-Engine,"  Popular 
Science  Monthly,  Vol.  XII,  p.  616,  Art.  "Liquefaction  of  Gases;" 
Loomis's  "Meteorology,"  and  Maury's  "Physical  Geography  of  the 
Sea." 


IX. 

ON  ELECTRICITY. 


That  power  which,  like  a  potent  spirit,  guides 

The  sea-wide  wanderers  over  distant  tides. 

Inspiring  confidence  where'er  they  roam, 

By  indicating  still  the  pathway  home  /  — 

Through  nature,  quickened  by  the  solar  beam 

Invests  each  atom  with  a  force  supreme, 

Directs  the  caverrid  crystal  in  its  birth, 

And  frames  the  mightiest  motintains  of  the  earth  ; 

Each  leaf  and  flower  by  its  strong  law  restrains, 

And  binds  the  monarch  Man  within  its  mystic  chains" 

HUNT. 


ANALYSIS. 


' 

{(1.)  Different  kinds  of. 

(2.)  The  poles. 

a 

(3.)  Magnetic  curves. 

UJ 

_J 

(4.)  Magnetic  needle. 

UJ    . 

(5.)  Attraction  and  repulsion. 

O  h- 

2.  INDUCTION. 

HO    ^ 

3.  To  MAKE  A  MAGNET. 

zE 

4.  THE  COMPASS. 

•T 

5.  POLARITY  OF  /  ^  Why  the  needle  points  north 

___             "\rT71TjlTVT    "E1             •                                              HJIQ.     SOlllil. 

'   [  (2.)  Dipping  needle. 
6.  MAGNETISM  OF  THE  EARTH. 

*   1.  ELECTROSCOPE. 

2.  Two  KINDS  OF  ELECTRICITY. 

3.  THEORY  OF  ELECTRICITY. 

>^ 

4.  CONDUCTORS  AND  INSULATORS. 

H 

5.  ELECTRICAL  MACHINE. 

oc 

f- 

6   INDUPTTOK       I  ^-)  Definition  and  illustration. 
ON>       1  (2.)  Faraday's  theory. 

c^ 

LU 

7.  ATTRACTION    J  (1.)  Electric  chimes. 

_J 
LU 

AND  REPULSION.  \  (2.)  Dancing  image. 

_| 

8.  LEYDEN  JAR. 

-< 

•^_ 

9.  ELECTRICITY  .RESIDES  ON  THE  SURFACE. 

O 

10.  EFFECT  OF  POINTS. 

H 

(  (1.)  Definitions. 

0 

11.  LIGHTNING.     -I  (2.)  Aurora  Borealis. 

cc 

ll 

[  (3.)  Lightning-rods. 

12.  VELOCITY  OF  ELECTRICITY. 

oi 

13.  EFFECTS    OF  J  W  j^siPalj 

>       ELECTRICITY.  |  ffi  ^SgicaL 

• 

1.  SIMPLE  GALVANIC  CIRCUIT. 

UJ 
I 

2.  SOURCE  OF  THE  HYDROGEN. 

ui 

3.  GALVANIC  CURRENT. 

^h- 

4.  ELECTRICAL  POTENTIAL. 

z  o  •< 

—6.  BATTERIES. 

>E 

6.  QUANTITY  AND  INTENSITY. 

5!*- 

7.  COMPARISON  OF  FRIC.  WITH  GALVANIC  ELEC. 

0 

CO 

8.  EFFECTS    OF  f  (1.)  Physical. 
GALVANIC  •{  (2.)  Chemical. 
ELECTRICITY.  [  (3.)  Physiological. 

1.  INFLUENCE  OF  A  CURRENT  ON  A  NEEDLE. 

6s 

.2.  GALVANOMETER. 

occo 

3.  ELECTRO-MAGNETS. 

tfc,  < 

4.  MOTION  PRODUCED  BY  ELECTRICITY. 

5.  MAGNETO  ELECTRICITY. 

Sg 

6.  INDUCED  CURRENTS. 

7.  THE  TELEPHONE. 

8.  THE  MICROPHONE. 

5.  THERMAL  ELECTRICITY. 

6.   ANIMAL  ELECTRICITY. 

ELECTRICITY. 

Electrical  energy  manifests  itself  in  several  different 
forms,  Magnetic,  Frictional,  Voltaic,  Thermal  and  Animal 
Electricity.  These  are  intimately  connected ;  their  laws  are 
strikingly  related ;  they  produce  many  effects  in  common ; 
and  one  can  give  rise  to  another. 

1.    MAGNETIC    ELECTMCITY. 

1.    Magnets.* — A  natural  magnet   is   an  ore  of  iron 
(Fe304,  Chemistry,  p.  154),  generally  known  as  the  load- 
stone (Saxon,  laedan,  to  lead).     The  artificial  magnet  is  a 
magnetized  steel  bar ;  if  straight,  it 
is  called  a  bar  magnet ;  if  U-shaped        Fl0m  188> 
a  horse-shoe  magnet.     A  piece  of 
soft  iron,  the  armature,  is  placed 
on  the  end. 

If  we  insert  a  magnet  in  iron- 
filings,  they  will  cling  chiefly  to  its 
ends  termed  the  poles.     The 
magnetic    force  will    be 
exerted  even  through 
an  intervening  bo$y. 
Lay  a  sheet  of  paper 
on    a    magnet    and 
sprinkle  iron-filings 
upon  it.    Gently  tap 
the  paper  and  theyi 
will  collect  in  curi-l 
ous  lines,  the  mag- 
netic curves,  radiat- 
ing from  the  poles. 
If  a  small  bar  magnet  be  suspended  so  as  to  swing  freely  in 

*  The  term  is  derived  from  the  fact  that  an  ore  of  iron  possessing  this  property 
was  first  found  at  Magnesia,  in  Asia  Minor. 


ELECTRICITY. 


Fie.  189. 


a  horizontal  plane,  one  pole  will  point  toward  the  north  and 

the  other  toward  the 
south.     The  north 
pole  of  the  magnet 
is  called  the  positive 
(  + ),  and  the  south 
pole,   the    negative 
( — ).       A    magnet 
thus  poised  consti- 
tutes a  magnetic 
needle.     If 
we  hold  a 
magnet 
near    a 

magnetic  needle,  we  shall  find  that  the  south  pole  of  one 
attracts  the  north  pole,  and  repels  the  south  pole  of  the 
other.*  This 

proves  the  law —  Fie.  190, 

' ( Like  poles  re- 
pel, and  unlike 
poles  attract." 

2.  Induction  is 
the  power  a  magnet 
possesses  to  develop 
magnetism  in  iron. 
If  a  piece  of  soft 
iron  be  brought 
near  a  magnet,  it 

*  Experiments.  1.  Rub  the  point  of  a  sewing-needle  across  the  north  pole  of  a 
magnet.  Bring  the  point  near  the  south  pole  of  the  magnetic  needle.  The  needle 
will  be  repelled,  showing  that  the  point  of  the  sewing-needle  has  become  a  south  pole. 
2.  Suspend  a  key  from  the  north  pole  of  a  magnet.  Bring  the  south  pole  of  an  equal 
magnet  close  to  the  upper  end  of  the  key.  The  key  will  instantly  fall.  3.  Suspend  a 
long  iron-wire  from  the  north  pole  of  a  magnet.  Bring  the  north  pole  of  the  second 
magnet  near  the  lower  end  of  the  wire.  The  wire  is  repelled,  because  its  lower 
extremity  possesses  north  polarity.  4.  Immerse  the  unlike  poles  of  two  magnets  in 
iron-filings.  Bring  the  two  poles  near  each  other.  The  filings  will  move  toward  one 
another.  But  if  the  poles  of  the  magnets  are  like,  the  filings  will  fall  off  the 
magnets.  5.  To  ascertain  whether  a  metallic  substance  contains  iron :  Bring  the 
substance  near  one  of  the  extremities  of  a  magnetic  needle.  If  the  position  of  the 


MAGNETIC    ELECTRICITY.  213 

immediately  assumes  the  magnetic  state,  but  loses  it  on 
being  removed.  In  steel  the  change  is  permanent.  The 
end  of  the  bar  next  to  the  south  pole  of  the  magnet  becomes 
the  north  pole  of  the  new  magnet,  and  vice  versa.  When 

opposite  states  are  thus 

' 191"       ^^  developed  in  the  oppo- 

site ends  of  a  body,  it 
is  said  to  be  polarized. 
Whenever  an  object  is 
attracted  by  a  magnet, 
it  is  supposed  first  to 

be  made  a  magnet  (polarized)  by  induction,  and  then 
the  attraction  consists  in  that  of  unlike  poles  for 
each  other.  Thus  we  may  suspend  from  a  magnet  a 
chain  of  rings  held  together  by  magnetic  attraction.* 
Eacli  link  is  a  magnet  with  its  north  and  south  poles.  Each 
particle  of  the  tuft  of  filings  in  Fig.  188  is  a  distinct  mag- 
net. By  inducing  magnetism  a  magnet  does  not  lose  force. 
It  rather  gains  by  the  reflex  influence  of  the  new  magnet. 
An  armature  acts  in  this  manner  to  strengthen  a  magnet, 
[f  we  break  a  magnet,  the  smallest  fragment  will  have  a 
north  and  a  south  pole.  This  is  explained  by  supposing 
that  every  molecule  contains  two  kinds  of  magnetism 
which  neutralize'  each  other.  When  magnetized  they  are 
separated,  but  do  not  leave  the  molecule  which  is  thus 
polarized,  the  halves  assuming  opposite  magnetic  states,  as 
shown  in  Fig.  192.  The 

light  half  of  each  little  cir-       E">-  m- 

cle  represents  the  positive, 

and   the  dark  the  negative 

side.     All  the  molecules  exert  their  negative  force  in  one 

direction,  and  their  positive  in  the  other.     The  forces  thus 

neutralize  each  other  at  the  centre,  but  manifest  themselves 

at  the  ends  of  the  magnet. 

needle  be  affected,  then  the  substance  almost  certainly  contains  iron.  A  piece  of 
copper  will  not  affect  the  magnetic  needle. 

*  Repeat  this  experiment  with  keys,  or  nails  of  different  sizes,  or  bits  of  wire  pf 
varying  length. 


214 


ELECTRICITY. 


3.  How  to  make  a  Magnet. — Place  the  inducing 
magnet,  as  shown  in  Fig.  193,  on  the  unmagnetized  one 
(which  any  blacksmith  can  make  from  a 
bar  of  steel),  and  draw  it  from  one  end 
to  the  other  several  times,  always  carry- 
ing it  back  through  the  air  to  the  start- 
ing-point.* 


PIG.  193. 


—  4:.  The  Compass  is  a  magnetic  needle 

^5J      used  by  mariners,  surveyors,  etc.     It  is 
""         delicately  poised  over  a  card  on  which  the 
"points  of  the  compass"  are  marked.     At  most  places  the 


FIG.  194. 


needle  does  not  point  directly  N.  and  S.     The  "line  of  no 
variation "  in  the  United  States  passes  (1885)  near  George- 

*  A  needle  may  be  magnetized  "by  laying  it  across  the  poles  of  a  horse-shoe  mag- 
net. After  remaining  a  time  the  end  in  contact  with  the  north  pole  of  the  magnet 
\viil  become  a  south  pole  and  the  other  a  north  pole.  If  it  be  suspended  from  ihe 
middle  by  a  thread  it  will  point  north  and  south.  A  knife-blade  may  be  magnetized 
by  rubbing  it  several  times,  in  the  same  direction,  across  one  of  the  poles  cf  the 
magnet. 


MAGNETIC   ELECTEICITY. 


215 


town,  S.  C.,  Zanesville,  0.,  and  Detroit,  Mich.  East  of  this, 
the  variation  (declination)  is  toward  the  west,  and  west  of  it 
the  declination  is  toward  the  east.* 


FIG.  195. 


FIG.  196. 


5.  Polarity  of  the  Needle. — WHY  THE  NEEDLE  POINTS 
NOBTH  AND  SOUTH. — The  earth  is  a  great  magnet.  This 
gives  direction  to  the  needle.  Variations  which  are  con- 
stantly taking  place  in 
the  terrestrial  magnetism 
produce  corresponding 
changes  in  the  needle. 
Suppose  a  magnet  NS 
passing  through  the  cen- 
tre of  a  small  globe. 
The  needle  sn  will  hang 
parallel  to  it  (Fig.  195), 
its  north  pole  being  at- 
tracted by  the  south  pole 
of  the  magnet,  and  vice 

versa.     If  the  globe  be  revolved  (Fig.  196),  the  north  pole  of 
the  needle  will  turn — dip,  as  it  is  termed — 
FIG.  197.  downward.     If  the  globe  be  revolved  in 

the  other  direction,  the  south  pole  of  the 
needle  will  dip  in  the  same  manner,  f 

A  DIPPING-NEEDLE  is  poised  as  shown 
in  Fig.  197.  At  the  magnetic  equator  it 
hangs  horizontally,  but  declines  when 
carried  north,  until,  at  a  point-  called 


*  The  declination  and  dip  have  daily  and  yearly  variations,  as  well  as  those  which 
require  centuries  to  complete.  The  needle  is,  however,  "  true  to  the  pole,"  although 
it  shifts  thus  every  hour  in  the  day.  It  does  so  only  in  obedience  to  the  laws  which 
control  its  action. 

t  Similar  phenomena  are  noticed  in  the  compass.  At  the  magnetic  equator  it  is 
horizontal,  but  dips  whenever  taken  north  or  south.  An  unmagnetized  needle,  if 
poised,  in  our  latitude,  on  being  magnetized,  settles  down,  as  if  the  north  end  were 
the  heavier.  This  is  remedied  by  making  the  north  end  of  the  needle  lighter,  and 
also  by  suspending  a  little  weight  upon  the  south  end.  The  reverse  is  true  in  the 
southern  hemisphere. 


216  ELECTRICITY. 

the  North  magnetic  pole,  on  Boothia  Peninsula,  it  becomes 
vertical.  * 

6.  Magnetism  of  the  Earth. — All  iron  bars  standing 
vertically  (in  this  latitude  not  far  from  the  line  of  the  dip) 
possess  slight  magnetic  properties.  Iron  fences,  lightning- 
rods,  iron  standards  of  chairs  and  desks,  pokers,  tongs, 
crow-bars,  etc.,  on  being  tested  by  the  magnetic  needle,  will 
be  found  to  possess  north  polarity  in  the  end  next  the 
ground,  and  south  polarity  in  the  other. 

2.    FEICTIONAL    ELECTKICITY. 

This  is  electricity  developed  by  friction.  Ex. :  One's  hair 
often  crackles  under  a  gutta-percha  comb.  A  cat's  back, 
when  rubbed  in  a  dark  room,  emits  sparks,  f 

1.  The  Electroscope  is  an  instrument  for  detecting 
the  presence  of  electricity.  Bend  a  glass  tube,  and  suspend 
from  it  by  silk  threads  a  couple  of  elder-pith  balls,  as 
shown  in  Fig.  199 ;  or  put  an  egg  in  a  wine-glass,  and 
balance  on  the  egg  a  dry  lath.  Two  strips  of  gold-leaf, 

*  The  magnetic  pole  is  now  nearly  in  the  longitude  of  Omaha.  In  New  York 
city,  the  dip  of  the  magnetic  needle  is  72°  11'  (1884) ;  and  the  declination  is  8°  west. 
The  former  is  decreasing  annually  1'.9 ;  and  the  latter  is  increasing  annually  2' .6. 
At  Washington,  the  declination  was  a  maximum  east  in  1796,  whereas  the  dip  was  a 
maximum  in  1859.  Each  place  has  its  own  magnetic  history. 

t  In  cold,  frosty  weather,  a  person,  by  shuffling  about  in  his  stocking-feet  upon 
the  carpet,  can  develop  so  much  electricity  in  his  body  that  he  can  ignite  a  jet  of  gas 
by  simply  applying  his  finger  to  it.— Blasts  in  mines  intended  to  be  fired  by  elec- 
tricity have  thus  been  prematurely  discharged  by  the  workmen  touching  the  wires. 
To  prevent  this  disastrous  effect,  at  the  Sntro  Tunnel,  Nevada  City,  the  workmen 
who  are  handling  exploders  wet  their  boots,  stand  on  an  iron  plate  to  conduct  ofl  the 
electricity  of  the  body,  and  wear  rubber  gloves. 


FBICTIOtfAL    ELECTRICITY,  817 

hung  in  a  glass  jar  (Fig.  200), 
form  a  test  so  sensitive,  that  a 
slight  flap  of  a  silk  handkerchief 
against  the  coyer  will  cause  the 
leaves  to  diverge. 

2.  Two  Kinds  of  Elec- 
tricity.— If  a  warm,  dry  glass 
tube  (a  lamp-chimney  will  an- 
swer) be  rubbed  with  a  silk 
handkerchief,  a  crackling  sound 
will  be  heard.  If  the  tube  be  held  near  the  face,  we  shall 
experience  a  sensation  like  that  of  a  cobweb.  The  tube  will 
attract  bits  of  paper,  straw,  feathers,  etc.*  Present  it  to 

*  The  following  simple  experiments  are  instructive  :  1.  A  rubber  comb  passed  a 
few  times  through  the  hair  will  furnish  enough  electricity  to  turn  the  lath  entirely 
around,  and  empty  egg  shells,  paper  hoops,  etc.,  will  follow  the  comb  over  the  table 
in  the  liveliest  way.  2.  Take  a  thin  sheet  of  gutta-percha,  about  a  foot  square  ;  lay 
H  upon  the  table,  and  rub  it  briskly  a  few  times  with  an  old  fur  cuff;  the  gutta-percha 
will  become  powerfully  electrified.  3.  Lift  the  gutta-percha  by  one  corner,  and  some 
force  will  be  required  to  separate  it  from  the  table.  4.  Hold  the  electrified  gutta- 
percha  in  the  left  hand ;  bring  the  fingers  of  the  right  near  the  paper ;  it  will  be 
attracted  to  the  hand,  and  sparks  will  pass  to  the  fingers  with  a  snapping  sound. 
5.  Hold  some  feathers,  suspended  by  a  silk  thread,  near  the  excited  gutta-percha  and 
the  feathers  will  be  attracted.  6.  Hold  the  excited  paper,  or  the  excited  sheet  of 
gutta-percha,  over  the  head  of  a  person  with  dry  hair  ;  the  hair  will  be  attracted  by 
the  gutta-percha,  and  each  particular  hair  will  stand  on  end.  7.  Hold  the  excited 
gutta-percha  near  the  wall ;  the  gutta-percha  will  fly  to  it,  and  remain  some  minutes 
without  falling.  8.  Place  a  sheet  of  gutta-percha  on  a  tea-tray ;  rub  the  gutta-percha 
briskly  with  a  fur  cuff:  place  the  tea-tray  with  the  excited  sheet  of  gutta-percha  on  a 
dry  tumbler  ;  lift  off  the  gutta-percha  from  the  tea-tray  :  bring  the  knuckle  of  your 
hand  near  the  tray,  and  you  will  receive  a  spark.  Replace  the  gutta-percha  on  the 
tray  and  apply  your  knuckle,  and  you  will  receive  another  spark.  This  may  be 
repeated  a  dozen  times.  9.  Take  a  sheet  of  foolscap  paper  and  a  board  about  the 
same  size.  Heat  both  till  they  are  thoroughly  dry.  While  hot,  lay  the  paper  on  the 
board  and  rub  the  former  briskly  with  a  piece  of  rubber.  The  paper  and  board  will 
cling  together.  Tear  the  paper  loose  and  try  experiments  4,  5,  6  and  7.  Return  the 
paper  and  rub  as  before.  Cut  the  paper  so  as  to  form  a  tassel.  Then  lift,  and  the 
strips  of  the  tassel  will  repel  one  another.  10.  Take  a  piece  of  common  brown  paper, 
about  the  size  of  an  octavo  book,  hold  it  before  the  fire  till  quite  dry  and  hot,  then 
draw  it  briskly  under  the  arm  several  times,  so  as  to  rub  it  on  both  sides  at  once  by 
the  coat.  The  paper  will  be  found  so  powerfully  electrical,  that  if  placed  against  a 
wainscoted  or  papered  wall  of  a  room,  it  will  remain  there  for  some  minutes  without 
falling.  11.  While  the  paper  still  clings  to  the  wall  hold  against  it  a  light,  fleecy 
feather  and  it  will  be  attracted  to  the  paper,  in  the  same  way  the  pap  er  is  to  the  wall. 
12.  If  the  paper  be  warmed,  drawn  under  the  arm  as  before,  and  then  hung  up  by  a 
thread  attached  to  one  corner,  it  will  sustain  several  feathers  on  each  side  ;  should 
these  fall  off  from  different  sides  at  the  same  time,  they  will  cling  together  very 

10 


218 


ELECTEICITY. 


the  pith-balls  in  the  electroscope.  They  will  be  attracted 
for  an  instanfc,  and  will  then  fly  from  the  tube  and  from 
each  other,  apparently  in  disgust.  Electrify  a  stick  of  seal- 
ing-wax and  present  it  to  the  balls.  They  will  act  in  the 

same  manner.     If  we  touch 
Fl°-  m  one  ball  with  the  excited  glass, 

and  the  other  with  the  ex- 
cited wax,  they  will  not,  as 
before,  fly  from  each  other,  but 
will  rush  together.  Present 
the  glass  to  a  ball ;  it  will  fly 
off  when  electrified.  Present 
the  glass  again,  and  it  will  be 
repelled.  Substitute  the  wax, 
and  it  will  be  attracted. 
Offer  now  the  glass,  and  it 
will  eagerly  bound  toward 
what  it  just  before  spurned. 

strongly ;  and  if  after  a  minute  they  are  all  shaken  off,  they  will  fly  to  one  another  in 
a  singular  manner.  13.  Warm  and  excite  the  paper  as  before,  and  then  lay  on  it  a 
ball  of  elder-pith,  about  the  size  of  a  pea  ;  the  ball  will  immediately  run  across  the 
paper,  and  if  a  needle  be  pointed  toward  it,  it  will  again  run  to  another  part,  and  so 
on  for  a  considerable  time.  14.  Support  a  pane  of  glass,  well  dried  and  warmed, 
upon  two  books,  one  at  each  end,  and  place  some  bran  underneath ;  then  rub  the 
upper  side  of  the  glass  with  a  silk  handkerchief,  or  a  piece  of  flannel,  and  the  bran 
will  dance  up  and  down  like  the  images  in  Fig.  207.  15.  Place  a  common  tea-tray  on 
a  dry,  clean  tumbler.  Then  take  a  sheet  of  foolscap  writing-paper  (as  in  No.  9)  and 
dry  it  carefully  until  all  its  hygrometric  moisture  is  exjjelled.  Holding  one  end  of  the 
sheet  on  a  table  with  the  finger  and  thumb,  rub  the  paper  with  a  large  piece  of  india- 
rubber  a  dozen  times  vigorously  from  left  to  right,  beginning  at  the  top.  Now  take 
up  the  sheet  by  two  of  the  corners  and  bring  it  over  the  tray,  and  it  will  fall  like  a 
stone.  This,  as  well  as  the  apparatus  in  No.  8,  forms  a  simple  Electrophones,  fit 
to  perform  many  experiments  ordinarily  performed  with  that  instrument.  If  the 
tip  of  a  finger  be  held  close  to  the  bottom  of  the  tray,  a  sensible  shock  will  be  felt. 
Next  lay  a  needle  on  the  tray  with  its  point  projecting  outward,  remove  the  paper, 
and,  in  the  dark,  a  star  sign  of  the  negative  electricity  will  be  seen ;  return  the 
paper,  and  the  positive  brush  will  appear.  Lay  a  dry,  hot  board,  as  in  No.  9,  on 
top  of  four  tumblers.  If  a  boy  stand  on  the  board  he  will  be  insulated,  and  on  his 
holding  the  tray  vertically,  the  paper  will  not  falL  Sparks  may  then  be  drawn 
from  his  body  and  his  hair  will  be  electrified  as  described  on  p.  230.  16.  Warm  a 
lamp-chimn  »;  rub  it  with  a  hot  flannel,  and  then  bring  a  downy  feather  near  it.  On 
the  first  moment  of  contact,  the  feather  will  adhere  to  the  glass,  but  soon  after  will 
fly  rapidly  away,  and  you  may  drive  it  about  the  room  by  holding  the  glass  between 
it  and  the  surrounding  objects  ;  should  it,  however,  come  in  contact  with  anything 
not  under  the  influence  of  electricity,  it  will  instantly  fly  back  to  the  glass. 


FEICTIONAL    ELECTRICITY.  219 

If  the  glass  be  held  on  one  side  of  a  ball  and  the  wax  on  the 
other,  it  will  fly  between  the  two,  carrying  the  electricity 
back  and  forth.  From  this  we  conclude  that  (1),  there  are 
two  kinds  of  frictional  as  of  magnetic  electricity ;  and  (2), 
like  electricities  repel  each  other,  and  unlike  attract.  The 
electricity  from  the  glass  is  termed  vitreous  or  positive  [  +  ], 
and  that  from  the  wax,  resinous  or  negative  [— ].* 

3.  Theory  of  Electricity.— Of  the  nature  of  electricity 
we  know  little.     The  positive  and  negative  forces,  or  fluids, 
as  they  are  often  styled,  exist  in  every  body  in  a  state  of 
equilibrium.     When  this  is  disturbed  by  friction,  chemical 
action,  etc.,  both  are  set  free.     We  cannot  develop  one 
without  the  other.     The  opposite  kinds  manifest  themselves 
at  opposite  parts  of  the  surface,  as  in  a  magnet ;  it  is  there- 
fore &  polar  force.     The  slightest  cause  disturbs  the  electric 
equilibrium.     "  In  cutting  a  slice  of  meat,  there  may  pass 
between  the  steel  knife  and  the  silver  fork  enough  elec- 
tricity to  move  the  needle  of  a  telegraph."    Yet  the  delicate 
balance  of  the  opposing  forces  is  so  soon  readjusted  that  w<? 
are  unconscious  of  the  change. 

4.  Conductors  and  Insulators. — A  body  which  allows 
the  electric  force  to  pass  through  it  freely  is  termed  a  con- 
ductor ;  one  which  does  not,  is  called  a  non-conductor,  or 
insulator.     Copper  is  one  of  the  best  conductors,  and  hence 
it  is  used  in  all  electrical  experiments.     Glass  is  one  of  the 
best  insulators.     A  body  is  said  to  be  insulated  when  it  is 
supported  by  some  non-conducting  substance,  usually  glass. 
Electricity  can  be  collected  only  by  insulation.     In  damp 
air  electricity  is  quickly  dissipated.     This,  according  to  the 
recent  experiments  of  Win.  Thomson,  is  due  to  the  deposit, 
on  the  glass  insulators,  of  a  thin  film  of  moisture,  which 

*  In  the  following  list,  each  substance  becomes  positively  electrified  when  rubbed 
with  the  body  following  it ;  but  negatively,  with  the  one  preceding  it.    (Cfanot.) 

1.  Cat's  fur.  5.  Cotton.  9.  Shellac.  13.  Caoutchouc. 

2.  Flannel.  6.  Silk  10.  Resin.  14.  Gutta-percha. 

3.  Ivory.  7   The  hand.  11.  The  metals.          15.  Gun-cotton 

4.  Glass.  8.  Wood.  12.  Sulohur. 


220 


ELECTRICITY. 


conducts  away  the  electricity.  Contrary  to  the  common 
statement,  there  is  no  difference  between  dry  and  damp  air 
as  an  insulator.* 

5.  The  Electrical  Machine  consists  (1)  of  a  glass 
wheel  turned  by  a  crank  ;  (2)  of  a  pair  of  rubbers  covered 
with  leather  and  spread  with  an  amalgam  (a  mixture  of  tin, 
zinc,  and  mercury)  which  hastens  the  development  of  elec- 
tricity ;  (3)  of  a  comb  or  fork  with  fine  points,  since  pointed 

Ite.  SOL 


bodies  always  favor  the  reception  or  dispersion  of  electricity  ; 
(4)  of  a  prime  conductor — a  brass  cylinder  insulated  by  a 
glass  standard  so  that  the  electricity  cannot  pass  to  the 


*  The  following  list  contains  the  most  common  conductors  and  insulators  : 
Best  Conductors.  Best  Insulators. 


Metals. 

Charcoal. 

Flame. 

Minerals. 

Water. 

Iron. 


Vegetables. 

Animals. 

Linen. 

Cotton. 

Dry  Wood. 

Ice. 


Shellac. 

Amber. 

Sulphur. 

Wax. 

Glass. 

Silk. 


Air  (dry  or  damp). 

Dry  Paper. 

Caoutchouc. 

Ice. 

Dry  Wood. 

Cotton. 


FRICTIONAL    ELECTRICITY.  221 

ground,  and  rounded  at  the  ends  so  that  it  may  not  escape 
too  rapidly  into  the  atmosphere. 

At  the  commencement,  the  whole  apparatus  is  in  a  state 
of  equilibrium.  By  friction,  positive  electricity  is  devel- 
oped on  the  glass,  and  negative  on  the  rubber.  The  neg- 
ative escapes  along  the  chain  to  the  ground — the  common 
reservoir.  The  positive,  kept  on  the  glass  by  the  silk  flaps, 
is  carried  around  to  the  points.  Here  it  polarizes  the  prime 
conductor,  i.  e.,  repels  its  positive  electricity  to  the  far  end 
and  attracts -its  negative  electricity  to  the  points;  the  two 
forces,  clashing  together,  form  tiny  sparks  and  are  neutral- 
ized.* The  positive  electricity  naturally  present  in  the  prime 
conductor  is  thus  left  insulated,  and  the  prime  conductor 
is  said  to  be  charged  with  positive  electricity.  If  the  neg- 
ative conductor  be  insulated,  the  rubber  will  soon  become 
charged  with  negative  electricity,  and  the  action  of  the 
machine  will  nearly  cease. 

HOLTZ'S  ELECTRICAL  MACHINE  is  shown  in  Fig.  202.  AB 
are  two  plate-glass  wheels,  the  former  fixed  and  the  latter 
made  to  revolve  by  turning  the  handle.  In  A  are  two 
openings  or  ivindows.  Near  these  are  glued  two  pieces  of 
varnished  paper  or  armatures,  //',  each  of  which  has  a 
point  projecting  through  the  window.  Opposite  the  arma- 
tures, and  on  the  other  side  of  the  revolving  wheel,  are 
two  rows  of  brass  points,  PP.  Connected  with  these  are 
insulated  conductors  terminating  in  the  movable  knobs  mn, 
the  poles  of  the  machine. 

In  starting,  the  poles  are  placed  in  contact,  and  one  of  the 
armatures  is  electrified  by  holding  against  it  a  piece  of 
excited  rubber  or  attaching  it  by  a  wire  to  the  conductor 
of  an  ordinary  electrical  machine.  On  then  separating  the 
poles,  a  brilliant  discharge  of  electricity  occurs.  The  usual 

*  ,  the  rubber  be  freshly  spread  with  amalgam,  and  the  glass  well  rubbed  with 
warm  flannel,  a  sharp  crackling  noise  will  be  heard,  flashes  will  follow  the  wheel, 
while  sparks  can  be  obtained  from  the  prime  conductor  at  a  distance  of  several 
inches.  The  pith-ball  electroscope,  when  charged  and  repelled  by  the  prime  con- 
ductor, will  be  quickly  attracted  by  the  rubber,  thus  indicating  their  opposite  eiec- 


ELECTKICITY. 


explanation  is  as  follows:  "The  two  armatures  are  elec- 
trified, one  positively  and  the  other  negatively.  The  pos- 
itive armature  (say/')  is  opposite  to  that  conductor  which 
gives  off  positive  electricity  from  its  knob  m,  and  the  pos- 
itive which  escapes  from  the  knob  is  replaced  by  positive 
which  is  drawn  off  by  the  points  from  the  face  of  the 
revolving  plate,  an  action  which  is  favored  by  the  inductive 

FIG.  202. 


influence  of  the  positively-charged  armature,  whether  we 
regard  it  as  repelling  positive  from  the  plate  to  the  brass 
points,  or  as  attracting  negative  from  the  conductor  through 
the  brass  points.  The  plate,  having  thus  given  off  pos- 
itive or  received  negative  electricity,  is  carried  round  through 
half  a  revolution,  and  gives  off  negative  to  the  other  con- 
ductor through  its  brass  points,  aided  by  the  influence  of 
the  negatively-charged  armature  at  the  back." 

6.   Induction.* — The  influence  of  an  electrified  body 

*  The  experiments  in  Fig.  203  can  be  nicely  performed  by  means  of  an  egg  placed 
flatwise  on  the  top  of  a  dry  wine-glass  and  the  glass  tube  used  on  p.  217.  Several  egga 
and  glasses  will  show  the  principle  of  Fig.  201  See  Tyndall's  Lessons  on  Eleo. 
tricity,  p.  39. 


FRICTIOKAL    ELECTRICITY. 


over  other  bodies  near  it  is  termed  electrical  induction.    Thus, 

let  an  insulated  conduc- 
FlG-  m  tor  be  placed  near  the  end 

of  the  prime  conductor 
of  an  electrical  machine. 
On  charging  the  prime 
conductor  the  motion  of 
the  pith-balls  shows  that 
the  small  conductor  has 
also  become  charged,  the 
end  next  the  positive 
prime  conductor  being 
negative,  and  the  other 
end  positive.  As  oppo- 
site electricities  are  thus 
developed  at  the  opposite 
extremities  of  the  conductor,  it  is  polarized.  Place  several 
conductors,  as  shown  in  Fig.  204,  connecting  the  copper 


ball  at  the  right  with  the  positive  pole,  and  the  one  at  the 
left  with  the  negative  pole  of  the  electrical  machine.  The 
conductors  will  be  charged  and  polarized  by  induction. 

FARADAY'S  THEORY  OF  INDUCTION-  assumes  (1)  that  the 
electricity  acts  between  the  different  molecules  of  a  body, 
as  between  the  different  conductors  in  the  last  experiment — 
that  each  molecule  becomes  polarized,  and  in  turn  polar- 
izes its  neighbors,  and  that  thus  at  last  every  molecule  has 
opposite  electricities  on  its  opposite  sides ;  (2)  that  the 


234 


ELECTRICITY. 


PIG.  205 


molecules  of  non-conductors  become  polarized  and  retain 
their  electricities,  while  the  molecules  of  conductors  become 
polarized  and  discharge  their  electricities  into  the  adjacent 
molecules.  The  positive  force  thus  accumulates  at  one  end, 
and  the  negative  at  the  other.  Let  P  (Fig.  205)  represent 
the  end  of  the  positive  conductor  and  N  that  of 
the  small  conductor  in  Fig.  203 ;  and  let  the 
small  circles  represent  molecules  of  air  lying  be- 
tween the  two — the  lighter  half  indicating  the 
positive  and  the  darker  half  the  negative  side. 
The  molecules  of  air  being  non-conducting,  on 
being  polarized  from  the  influence  of  P,  the  prime 
conductor,  retain  their  electricities,  but  polarize 
one  another  in  succession  until  N  is  reached. 

This  being  a  con- 
ducting body,  its  mol- 
ecules impart  their  electricity 
from  one  to  the  other,  until  the 
negative  electricity  collects  at 
one  end  and  the  positive  at  the 
other. 


FIG.  206. 


7.  Attraction  and  Repul- 
sion.— Every  case  of  attraction 
or  repulsion  is  preceded  by  in- 
duction. Ex.:  "The  electric 
chime"  consists  of  three  bells, 
two  of  which,  c  and  b,  are  hung  by  brass  chains,  while  the 
middle  one  is  insulated  above  by  a  silk  cord,  and  connected 
below  with  the  earth  by  a  chain.  The  balls  between  them 
are  also  insulated.  The  outer  bells  becoming  charged  with 
positive  electricity  from  the  prime  conductor,  polarize  the 
balls  by  induction  through  the  intervening  air.  The  balls 
being  then  attracted  to  the  bells,  are  charged  and  imme- 
diately repelled.  Swinging  away,  they  strike  against  the 
middle  bell,  discharging  their  electricity,  and  are  forthwith 
attracted  again.  Flying  to  and  fro,  they  ring  out  a  merry, 
electrical  song. 


FRICTIONAL    ELECTRICITY. 


225 


FIG.  207. 


The  dancing  image  consists  of  a  pith-ball  figure  placed 
between  two  metallic  plates,  the 
upper  one  hanging  from  the  prime 
conductor,,  and  the  lower  one  con- 
nected with  the  earth.  The  dance 
is  conducted  by  alternate  attrac- 
tion and  repulsion.* 


FIG.  208. 


8.  The  Leyden  Jarf  consists 

of  a  glass  jar  coated  inside  and 

outside,  nearly  to  the  top,  with 

tinfoil.     It  is  fitted  with  a  cover  of 

baked  wood,  through  which  passes 

a  wire  with  a  knob  at  the  top,  and 

below,  a  chain  extending  to  the  inner  coating.  The  jar  is 

charged  by  bringing  the  knob  near 
the  prime  conductor  of  the  elec- 
trical machine,  while  the  outer 
coating  communicates  with  the 
earth.  Bright  sparks  will  leap  to 
the  inner  coating,  while  similar 
ones  will  pass  off  from  the  outer 
coating.  The  jar  is  discharged  by 
holding  one  knob  of  the  "dis- 
charger "  E,  upon  the  outer  coat- 
ing, and  the  other  upon  the  knob 
of  the  jar.  The  equilibrium  will 

be  restored  with  a  sharp  snap  and  a  brilliant  flash.     Minute 


*  A  slow  motion  should  be  given  to  the  electrical  wheel,  and  a  pin  thrust  into  the 
heel  of  the  image  will  add  much  to  the  stamp  of  the  tiny  feet. 

t  It  is  said  that  Cnneus,  a  pupil  at  Leyden,  discovered  the  principle  of  the  Leyden 
jar  in  the  following  curious  way:  While  experimenting,  he  held  a  bottle  of  water  to 
the  prime  conductor  of  his  electrical-machine.  Noticing  nothing  peculiar,  he  at- 
tempted to  investigate  its  condition.  Holding  the  bottle  with  one  hand,  he  happened 
to  touch  the  water  with  the  other,  when  he  received  a  shock  so  unexpected,  and  so 
unlike  anything  he  had  ever  felt  before,  that  he  was  filled  with  astonishment.  It  was 
two  days  before  he  recovered  from  his  fright.  A  few  days  afterward,  in  a  letter  to  a 
friend,  the  physicist  innocently  remarked,  that  he  would  not  take  another  shock  for 
the  whole  kingdom  of  France. 


236  ELECTRICITY. 

particles  detached  from  the  solid  conductors  burn,  giving 
color  and  brilliancy  to  the  spark.* 

Explanation. — The  essentials  of  the  jar  are  two  conduct- 
ing surfaces  separated  by  a  non-conducting  body.    The  tin- 
foil acts  only  as  a  conductor  to  convey  the  electricity,  f    The 
jar  is  charged  on  the  same  principle  as  the  prime  conductor 
of  the  electrical  machine.    The  positive 
prime  conductor  draws  out  the  negative 
electricity  of  the  inner  coating,  leaving 
it  charged  with  positive.    The  molecules 


pec* 


p€>€* 

locc« 


oce* 


of  glass  are  polarized  (Fig.  209).  A  like 
quantity  of  positive  electricity  then  escapes 
from  the  outer  coating.  If  the  jar  be  in- 
sulated so  that  this  is  unable  to  leave,  the 
passage  of  the  sparks  will  cease.  If  a  finger 


be  held  near  the  outer  coating,  a  spark  will 
leap  to  it  every  time  one  enters  the  jar.  The  jar,  therefore, 
when  charged,  contains  no  more  electricity  than  in  its  natural 
state.  It  is  only  differently  distributed. 

9.  Electricity  is  on  the  Surface. — Each  molecule 
within  the  surface  of  a  solid,  insulated  conductor  gives  up 
its  electricity  with  equal  freedom  in  every  direction ;  there- 
fore it  cannot  become  charged.    Each  molecule  on  the  sur- 
face, however,  receiving  electricity  from  the  particles  behind 
it,  and  having  non-conducting  particles  of  air  before  it, 
must  become  charged.    A  bomb-shell  can  therefore  hold  as 
much  electricity  as  a  cannon-ball. 

10.  Effect  of  Points.— A  pointed  wire  held  near  the 
prime  conductor  will  quietly  draw  off  all  its  electricity, 

*  The  incredibly  small  quantity  of  the  metal  volatized  in  this  way  is  a  striking 
proof  of  the  divisibility  of  matter.  During  some  experiments  at  the  Philadelphia 
mint  a  gold  pole  lost  in  weight  by  a  strong  spark  one  millionth  of  a  grain ;  and 
nA&joa  of  a  grain  of  nickel  signed  its  name  in  the  spectroscope  brilliantly.  See  Popu- 
lar Science  Monthly,  May,  1877. 

t  This  Is  illustrated  by  the  "  Leyden  Jar  with  movable  coatings,"  which  may  be 
charged  and  tLen  taken  apart.  Very  little  electricity  can  be  obtained  from  the  glass, 
the  tin  coatings,  or  any  two  of  the  parts  combined.  When  put  together  again,  the 
Jar  can  be  discharged  in  the  usual  manner. 


FKICTIOJffAL    ELECTKICITY.  227 

which  will  be  seen  apparently  clinging  to  the  point  like  a 
glowing  star.  If  we  fasten  a  pointed  wire  to  the  prime  con- 
ductor, it  will  silently  discharge  the  electricity  in  a  "  brush" 
of  flame.  If  we  hold  one  cheek  near  the  point,  we  shall 
feel  a  current  of  air  setting  away  from  it.  This  is  strong 
enough  to  deflect  the  flame  of  a  candle.  The  particles  of 
air  near  the  point  becoming  polarized,  are  attracted,  give 
up  their  negative  electricity,  and,  being  charged  with  posi- 
tive electricity,  are  repelled ;  new  ones  take  their  place,  and 
thus  a  current  is  established.* 

11.  Lightning  is  only  the  discharge  of  a  Leyden  jar  on 
the  grand  scale  upon  which  Nature  performs  her  operations. 
Two  clouds  charged  with  opposite  electricities,  and  sepa- 
rated by  the  non-conducting  air,  approach  each  other,  f  When 
the  tension  becomes  sufficient  to  overcome  the  resistance, 
the  two  forces  rush  together  with  a  blinding  flash  and  ter- 
rific peal.  The  lightning  moves  along  the  line  of  the  least 
resistance,  and  so  describes  a  zigzag  course.  If  we  can  trace 
the  entire  length,  we  call  it  chain-lightning ;  if  we  see  only 
the  flash  through  intervening  clouds,  it  is  sheet-lightning ; 
and  if  the  reflection  of  distant  discharges,  we  term  it  heat- 
lightning.  The  report  is  caused  by  the  clashing  of  displaced 
air.  The  rolling  of  the  thunder  is  produced  by  the  reflec- 
tion of  the  sound  from  distant  clouds.  Sometimes  the 
clouds  and  the  earth  become  charged  with  opposite  elec- 
tricities, separated  by  the  non-conducting  air.  The  spark 
from  the  discharge  of  this  huge  Leyden  jar  is  a  bolt  that 
often  causes  fearful  destruction. 


*  The  electric  whirl,  mounted  on  the  prime  conductor  (Fig.  201),  illustrates  this 
action.  As  each  molecule  of  air  is  repelled  from  a  point,  it  reacts  with  equal  force 
against  the  point.  This  is  sufficient  to  set  the  light  wire-wheel  in  rapid  rotation. 

t  The  air  is  almost  constantly  electrified  by  the  friction  of  moving  clouds,  winds, 
etc.,  by  heat  and  chemical  changes— all  of  which  disturb  the  equilibrium.  In  clear 
weather  it  is  in  a  positive  state,  but  in  foul  weather  it  changes  rapidly  from  positive 
to  negative,  and  vice  versa.  Dr.  Livingstone  tells  us  that  in  South  Africa  the  hot 
wind  which  blows  over  the  desert  is  so  highly  electrified,  that  a  bunch  of  ostrich 
feathers  held  for  a  few  seconds  against  it  becomes  as  strongly  charged  as  if  attached 
to  au  electrical  machine,  and  will  clasp  the  liaudjmh  a  sharp  crackling  sound. 


228  ELECTEICITY. 

The  AURORA  BOREALIS*  ("Northern  Lights")  is  prob- 
ably caused  by  the  passage  of  electricity  through  the  rare- 
fied atmosphere  of  the  upper  regions.  The  intimate  relation 
between  the  aurora  and  magnetism  is  shown  from  the  fact 


FIG.  210. 


that  the  magnetic  needle  is  disturbed  when  the  aurora  is 
visible,  and  seems  to  tremble  as  the  streamers  dart  to  and 
fro.  The  telegraph  has  been  worked  by  the  current  of 
electricity  which  passes  along  the  wire  on  these  occasions. 

LIGHTNING-RODS  were  invented  by  Franklin,  f  They  are 
based  on  the  principle  that  electricity  always  seeks  the  best 

*  It  may  be  beautifully  imitated,  on  a  email  scale,  by  passing  a  succession  of 
sparks  from  the  prime  conductor  through  the  rarefied  air  contained  in  a  long  g'ass 
tube,  shown  in  Fig.  29. 

t  Franklin's  plan  was  opposed  by  many  philosophers  of  the  day,  who  declared  it 
was  as  impious  to  ward  off  Heaven's  lightning, lk  as  for  a  child  to  ward  off  the  chas- 
tening rod  of  its  father."  There  was  much  discussion  as  to  whether  the  conductors 
should  be  pointed  or  not.  Wilson  persuaded  George  HI.  that  the  points  were  a 
republican  device  to  injure  His  Majesty,  as  they  would  certainly  "  invite  "  the  light- 
ning, and  so  the  points  on  the  lightning-rods  upon  Buckingham  Palace  were  changed 
for  balls. 


FBIOTIONAL    ELECTRICITY.  229 

conductor.  The  rod  should  be  pointed  at  the  top  with  some 
metal  which  will  not  easily  corrode.  If  constructed  in 
separate  parts,  they  should  be  securely  jointed.  The  lower 
end  should  extend  into  water,  or  else  deep  into  the  damp 
ground,  beyond  a  possibility  of  any  drought  rendering  the 
earth  about  it  a  non-conductor,  and  be  packed  about  with 
ashes  or  charcoal.  If  the  rod  is  of  iron,  it  needs  to  be 
much  larger  than  one  of  copper,  which  is  a  better  conductor. 
Every  elevated  portion  of  the  building  should  be  protected 
by  a  separate  rod.  Chimneys  need  especial  care,  because  of 
the  ascending  column  of  vapor  and  smoke.  Water  con- 
ductors, tin  roofs,  etc.,  should  be  connected  with  the  damp 
ground  or  the  lightning-rod,  that  they  may  aid  in  convey- 
ing off  the  electricity.* 

12.  Velocity  of  Electricity. — The  duration  of  the 
flash  has  been  estimated  at  one-millionth  of    a  second. 
Some  idea  of  its  instantaneousness  can  be  formed  from  the 
fact  that  the  spokes  of  a  wheel,  revolved  so  rapidly  as  to 
appear  confused  by  daylight,  can  be  distinctly  seen  by  the 
spark  from  a  Leyden  jar.     The  trees  swept  by  the  tempest, 
or  a  train  of  cars  in  rapid  motion,  when  seen  by  a  flash  of 
lightning,  seem  motionless ;   while  a  cannon-ball,  in  swift 
flight,  appears  poised  in  mid-air,  f 

13.  Effects  of  Frictional  Electricity. — (1.)   PHYS- 

*  The  value  of  a  lightning-rod  consists,  most  of  all,  in  its  power  of  quietly 
restoring  the  equilibrium  between  the  earth  and  the  clouds.  By  erecting  lightning- 
rods,  we  thus  lessen  the  liabilities  of  a  sudden  discharge.  Providence  has  guarded 
largely  against  this  catastrophe.  "  God  has  made  a  harmless  conductor  in  every  leaf, 
spire  of  grass,  and  twig.  A  common  blade  of  grass,  pointed  by  Nature's  exquisite 
workmanship,  is  three  times  more  effectual  than  the  finest  cambric  needle,  and  a 
single  pointed  twig  than  the  metallic  point  of  the  best-constructed  rod."  Every 
drop  of  rain,  and  every  snowflake,  falls  charged  with  the  electric  force,  and  thus 
quietly  disarms  the  clouds  of  their  terror.  The  balls  of  electric  light,  called  by 
sailors  "St.  Elmo's  fire"  which  sometimes  cling  to  the  masts  and  shrouds  of  vessels, 
and  the  flames  seen  to  play  about  the  points  of  bayonets,  indicate  the  quiet  escape 
of  the  electric  force  of  the  earth  toward  the  clouds. 

t  Recent  investigations  indicate  that  electricity  has  no  fixed  velocity,  but  that  its 
rate  depends  on  its  intensity  and  the  medium  through  which  it  passes.  Prof.  Rood, 
of  Columbia  College,  has  produced  sparks  whose  duration  was  forty -one  billionthe 
of  a  second. 


230  ELECTRICITY. 

ICAL. — Discharges  from  a  large  battery  of  Leyden  jars  will 
melt  metal  rods,  perforate  glass,  split  wood,  magnetize  steel 
bars,  etc. — Let  a  person  stand  upon  an  insulated  stool  and 
become  charged  from  the  prime  conductor.  His  hair, 
through  repulsion,  will  stand  erect  in  a  ludicrous  manner. 
On  presenting  his  hand  to  a  little  ether  contained  in  a  warm 
spoon,  a  spark  leaping  from  his  extended  finger  will  ignite 
it.  If  he  hold  in  his  hand  an  icicle,  the  spark  will  readily 
dart  from  it  to  the  liquid.* — A  card  held  between  the  knob 
of  a  Leyden  jar  and  that  of  the  discharger,  will  be  punc- 
tured by  the  spark. — A  piece  of  steel  may  be  magnetized  by 
the  discharge  from  a  Leyden  jar.  Wind  a  covered  copper 

FIG.  211. 


wire  around  a  steel  bar,  as  in  Fig.  211,  or  enclose  a  needle 
in  a  small  glass  tube,  around  which  the  wire  may  be  wound. 
FIG.  212.  On  passing  the  spark  through  the  wire, 

the  needle  will  attract  iron-filings. — When 
strips  of  tinfoil  are  pasted  on  glass,  and 
figures  of  various  curious  patterns  cut 
from  them,  the  electric  spark  leaping  from 
one  to  the  other  presents  a  beautiful  ap- 
pearance.— If  a  battery  be  discharged 
through  a  wire  too  small  to  conduct  the 
spark,  the  electricity  will  be  changed  to 
heat,  and  if  sufficiently  small,  the  wire 
will  be  fused  into  globules  or  dissipated  in  smoke. 

(2.)  CHEMICAL  EFFECTS. — The  "electric  gun"  is  filled 
with  a  mixture  of  oxygen  and  hydrogen  gases.  A  spark 

*  This  experiment  can  be  more  surely  performed  by  using  di-eulphide  of  carbon. 
The  insulating  stool  may  be  merely  a  board  laid  on  four  dry  flint-glass  bottles  or 
goblets,  and  the  electricity  be  developed  by  rubbing  a  glass  tube,  as  on  p.  217. 


VOLTAIC    ELECTRICITY.  231 

causes  them  to  combine  with  a  loud  explosion  and  form 
water. — The  sulphurous  smell  which  accompanies  the  work- 
ing of  an  electrical  machine,  and  is  noticed  in  places  struck 
by  lightning,  is  owing  to  the  production  of  ozone,  a  peculiar 
form  of  the  oxygen  of  the  air.  (See  Chemistry,  p.  38.) 

(3.  PHYSIOLOGICAL  EFFECTS. — A  slight  charge  from  a 
Leyden  jar  produces  a  contraction  of  the  muscles  and  a 
spasmodic  sensation  in  the  wrist.  A  stronger  one  becomes 
painful  and  even  dangerous. 

3.    VOLTAIC    ELECTRICITY* 

1.  Simple  Voltaic  Circuit,  f — If  we  place  a  strip  of 
zinc  in  water  acidulated  with  sulphuric 
acid  (oil  of  vitriol),  a  chemical  action 
will  at  once  commence.  Bubbles  of 
hydrogen  gas  gather  on  the  metal, 
while  the  zinc  rapidly  dissolves.  J 
Now  put  a  strip  of  copper  in  the 
liquid.  While  the  two  metals  remain 
separate  no  change  takes  place,  but  as 
soon  as  they  touch,  or  are  connected 
by  wires,  as  in  the  figure,  chemical 
action  begins,  and  the  bubbles  of  hy- 
drogen gather  upon  the  copper  instead  of,  as  before,  upon 
the  zinc.  The  copper  is  unchanged,  but  the  zinc  wastes 
away.  As  soon  as  the  wires  are  separated  the  action  ceases, 
and  when  they  are  rubbed  together  in  the  dark,  a  minute 
spark  is  seen. 

*  This  name  is  given  in  honor  of  the  Italian  philosopher  who  made  the  first  dis- 
coveries in  this  branch  of  electricity.  (See  p.  251.) 

t  We  can  easily  form  a  simple  galvanic  circuit  by  placing  a  silver  coin  between 
our  teeth  and  upper  lip,  and  a  piece  of  zinc  under  our  tongue.  On  pressing  the  edges 
of  the  two  metals  together,  a  peculiar  taste  will  be  perceived,  while  a  flash  of  light 
will  pass  before  the  closed  eyes. 

t  If  we  immerse  the  zinc  in  mercury,  the  surface  will  become  as  bright  as  a  mir- 
ror. Replace  the  strip  in  the  cup,  and  the  acid  will  have  no  effect  upon  it  as  long  aa 
the  current  does,  not  pass.  The  reason  of  this  action  is  not  well  understood.  Zino 
used  in  galvanic  batteries  is  thus  amalgamated. 


232  ELECTRICITY. 

Two  metal  plates  combined  in  this  manner  form  a  gal- 
vanic pair.  The  ends  of  the  wires  are  termed  poles  or  elec- 
trodes. The  copper  pole  is  positive  and  the  zinc  negative.* 
Joining  the  wires  is  termed  closing  the  circuit,  and  separating 
them,  breaking  the  circuit. 

2.  "Why  the  Hydrogen  comes  from  the  Copper. 

— For  simplicity  of    illustration,   we    suppose   a    row   of 
water  molecules  f  extending  from  the  zinc 
to  the  copper  plate.     The  negative  oxygen 
of  the  molecule  of  water  nearest  the  pos- 
itive zinc  is  attracted  to  that  plate,  while 
the  positive  hydrogen  is  repelled.      The 
atom  thus  driven  off  seeks  refuge  with  the 
oxygen  of   the  next    molecule,   and    dis- 
possesses its  hydrogen.     This  atom  in  turn 
robs  the  third  molecule  of  its  oxygen,  and 
so  on  until  the  last  molecule  is  reached, 
when  the  atom  of  hydrogen,  attracted  by  the  negative  cop- 
per, gives  up  to  it  its  positive  electricity,  and  then  flies  off 
into  the  air.     Each  atom  of  escaping  hydrogen  imparting 
its  electrical  force,  adds  to  the  current  of  electricity. 

3.  Voltaic  Current. — The  word  "current"  should  not 
be  understood  to  indicate  the  passage  of  a  fluid,  like  the 
flow  of  water  in  a  stream,  but  a  mere  transmission  of  the 
electrical  energy.     Thus,  if  a  long  pipe  were  perfectly  filled 
with  water,  a  drop  added  at  one  end  would  thrust  out  a 
corresponding  one  at  the  other,  which  would  not,  however, 

*  These  names  may  easily  be  remembered  if  we  associate  the  p's  with  copper 
and  positive,  and  the  n's  with  zinc  and  negative.  It  should  be  noticed  that  the  terms 
are  reversed  when  applied  to  the  plates  and  the  poles  The  zinc  pole  is  negative, 
but  the  zinc  plate  is  positive ;  the  copper  pole  is  positive,  but  the  copper  plate  is 
negative.  We  thus  see  that  the  plates  when  placed  in  the  liquid  become  polarized, 
as  is  represented  in  Fig.  213. 

t  In  Pig.  214  a  molecule  of  water  is  represented,  for  convenience,  as  consisting 
of  only  one  atom  of  hydrogen  and  one  of  oxygen,  instead  of  two  of  hydrogen  and 
one  of  oxygen.— Late  investigators  hold  that  the  hydrated  sulphuric  acid  (32SO4), 
rather  than  the  water  alone,  is  decomposed,  the  hydrogen  going  to  the  copper  and 
the  sulphuric  acid  combining  with  the  zinc.  (See  replacement  theory,  Chemistry^ 
p.  61.) 


VOLTAIC    ELECTRICITY.  233 

be  the  identical  one  dropped  in,  since  the  force  alone  would 
traverse  the  length  of  the  pipe.  In  the  galvanic  pair  the 
current  of  positive  electricity  sets  out  from  the  positive 
zinc  through  the  liquid  to  the  negative  copper,  thence 
through  the  wire  around  again  to  the  zinc.  If  the  circuit 
be  broken,  the  current  will  manifest  itself  at  the  copper 
pole.  When  the  current  passes  through  a  conducting  sub- 
stance, as  a  wire,  rod,  etc.,  the  force  is  transmitted,  not  on 
the  surface,  but  through  the  entire  thickness  of  the  lody. 
Each  molecule,  becoming  polarized  and  charged,  discharges 
its  force  into  the  next  molecule,  and  so  on.  The  current 
thus  moves  by  a  rapid  succession  of  polarizations  and  dis- 
charges  of  the  molecules  o±  the  conductor.* 

4.  Electrical  Potential  is  a  property  of  a  body  by 
which  electricity  tends  to  go  from  it  to  another,  and  is 
measured   by  the   resistance   met  in   the   passage.      This 
term  is  to  electricity  what  temperature  is  to  heat.     When 
plates  of  zinc  and  copper  are  brought  in  direct  contact, 
or  are  connected  by  means  oi  the  liquid,  they  assume  dif- 
ferent electrical  potentials  as  already  seen  (Fig.  213).     On 
joining  the  wires,  the  electricity  passes  from  the  body  at 
the  higher  potential  to  the  one  at  the  lower.      This  con- 
stitutes the  current,  which  is  made  a  steady  flow  by  the 
constant  chemical  decomposition  taking  place.     The  greater 
the  difference  between  the  chemical  action  upon  the  two 
metals  the  stronger  the  current.     The  metal  most  corroded 
is  termed  the  positive  or  generating  plate,  and  the  one  least 
corroded  the  negative  or  collecting  plate. 

5.  A  battery  consists  of  several  voltaic  pairs,  combined 
so  as  to  increase  the  strength  and  steadiness  of  the  electric 
current. 

*  With  what  inconceivable  rapidity  must  these  successive  changes  take  place  in 
an  iron  wire  to  transmit  the  electric  force,  as  in  recent  experiments,  from  Valentia, 
Ireland,  across  the  bed  of  the  Atlantic  and  the  continent  to  San  Francisco  and  return, 
a  distance  of  14,000  miles,  in  two  minutes  1 


234 


ELECTKICITT. 


PIG.  215, 


SMEE'S  BATTERY.  —  Each  cell  consists  of  two  plates  of 
zinc  and  one  of  silver  suspended  between 
them.  They  are  clamped  together  with 
screws  and  hung  in  a  glass  jar  filled  with 
dilute  sulphuric  acid.  Since  bubbles  of 
hydrogen  gas  tend  to  collect  on  the  smooth 
surface  of  the  silver  and  interrupt  the 
action,  it  is  roughened  with  finely-divided 
platinum.  (See  also  p.  253.) 

GROVE'S  BATTERY  is  a  "two-fluid  bat- 
tery." The  outer  or  glass  jar 
contains  dilute  sulphuric  acid, 
in  which  is  placed  a  hollow 
zinc  cylinder  with  a  slit  at  the  side  to  allow 
a  free  circulation  of  the  liquid.  The  inner 
cup  is  of  porous  earthenware,  and  filled  with 
strong  nitric  acid,  in  which  is  suspended  a  thin 
strip  of  platinum.* 


FIG.  216. 


BATTERY  differs  from  Grove's  in  substituting 
a  carbon  rod  for  the  platinum  strip  in  the  inner  cup.  This, 
being  an  excellent  conductor,  answers  the  same  purpose  and 
is  cheaper. 

DANIELL'S  CONSTANT  BATTERY  has  an  outer  copper  cup 

*  The  chemical  change  Is  as  follows  :  The  water  in  the  outer  cup  is  decomposed, 
the  oxygen  uniting  with  the  zinc  and  the  sulphuric  acid  with  both,  to  make  sulphate 
of  zinc.  The  hydrogen,  however,  does  not  escape,  as  in  Smee's  battery,  but  passes 
into  the  inner  cup  and  tears  apart  the  nitric  acid,  forming  water  and  nitric  oxide. 
(See  Chemistry,  p.  46.)  The  latter  is  at  first  absorbed  by  the  liquid,  but  soon  begins 
to  escape  in  corrosive,  reddish  fume?.  According  to  the  new  nomenclature  in 
Chemistry,  the  sulphuric  acid,  H2SO4,  is  decomposed,  the  hydrogen  passing  into  the 
inner  cup  and  the  anhydrous  acid  combining  with  the  zinc.  If  the  zinc  is  properly 
amalgamated,  no  action  takes  place  while  the  poles  are  separated,  and  the  battery 
remains  quiescent,  like  a  sleeping  giant,  but  the  instant  the  wires  are  connected  the 
liquid  will  begin  to  boil  with  the  evolution  of  the  gas,  while  the  electric  energy  is 
manifested  at  the  poles.  The  advantages  of  this  battery  are  :  (1.)  The  hydrogen  does 
not  collect  on  the  negative  (platinum)  plate,  since  it  is  absorbed  by  the  nitric  acid. 
(2.)  The  liquid  formed  in  the  inner  cup  is  an  excellent  conductor  of  electricity. 
(3.)  Platinum  is  a  more  perfect  negative  metal  than  copper,  since  it  is  not  acted  upon 
by  the  acid,  and  thus  does  not  tend  to  start  a  counter-current  ;  therefore  platinum 
and  zinc  make  a  better  voltaic  pair  than  copper  and  zinc.  (4.)  The  additional 
decomposition  of  the  nitric  acid  sets  free  a  great  quantity  ef  electricity. 


VOLTAIC    ELECTRICITY.  235 

holding  a  solution  of  blue  vitriol,  and  an  inner  porous  cup 
containing  a  zinc  rod  and  dilute  sulphuric  acid.  The  sul- 
phate of  copper  battery  consists  of  a  large  zinc  cylinder 
suspended  in  a  copper  jar  containing  a  solution  of  copper 
sulphate  (blue  vitriol).  (See  also  p.  253.) 

6.  Quantity  and.  Intensity. — A  battery  may  develop  a 
great  quantity  of  electricity  having  a  low  degree  of  intensity, 
or  a  small  quantity  having  a  high  intensity.     Thus  a  cup  of 
boiling  water  is  intensely  hot,  while  a  hogshead  of  luke- 
warm water  contains  a  great  quantity  of  heat.    The  in- 
tensity of  the  electric  force  depends  on  the  number  of  cells  ; 
the  quantity,  on  their  size.     An  intensity  battery  is  formed 
by  joining  the  zinc  plate  of  one  cell  to  the  copper  of  the 
next ;  a  quantity  battery,  by  joining  the  zinc  and  the  copper 
plates  together. 

7.  Comparison  of  Frictional  with   Voltaic   Elec- 
tricity.— Frictional  electricity  is  noisy,  sudden,  and  con- 
vulsive ;  voltaic  is  silent,  constant,  and  powerful.*     The  one 
is  a  quick,  violent    blow ;    the    other  a    steady,   uniform 
pressure.     Intensity  is   the   characteristic    of   the  former, 
quantity  of  the   latter.     The   lightning   will   leap  through 
miles  of  intervening  atmosphere,  while  the  voltaic  current 
will  follow  a  conductor  around  the  globe,  rather  than  jump 
across  the  gulf  of  a  half-inch  of  air.     The  most  powerful 
frictional  machine  would  be  insufficient   for  telegraphing; 
while  despatches  have  been  sent  across  the  ocean  with  a  tiny 
battery  composed  of  "  a  gun-cap  and  a  strip  of  zinc,  excited 
by  a  drop  of  water  the  bulk  of  a  tear." 

*  To  decompose  a  grain  of  water  would  require  over  6,000,000  discharges  from  a 
Leyden  jar— enough  electricity  to  charge  a  thunder-cloud  35  acres  in  area  ;  yet  a  few 
Grove  cups  would  tear  apart  that  amount  of  water  in  perfect  ease  aud  silence. 
"Faraday  immersed  a  voltaic  pair,  composed  of  a  wire  of  platinum  and  one  of  zinc, 
in  a  solution  of  4  ozs.  of  water  and  one  drop  of  oil  of  vitriol.  In  three  seconds  this 
produced  as  great  a  deviation  of  the  galvanometer  needle  (Fig.  220)  as  was  obtained 
by  30  turns  of  the  powerful  plate  glass  machine.  If  this  had  been  concentrated  in 
one  millionth  of  a  second,  the  duration  of  an  electric  spark,  it  would  have  been  suffi- 
cient to  kill  a  cat ;  yet  it  would  require  800,000  such  discharges  to  decompose  a  grain 
pf  water/' 


236 


ELECTRICITY. 


PIG.  217. 


8.  Effects  of  Voltaic  Electricity. — (1.)  PHYSICAL. 
If  a  current  of  electricity  is  passed  through  a  wire  too 
small  to  conduct  it  readily,  this  becomes  hot.  The 
poorer  the  conducting  power  of  the  wire,  and  hence  the 
greater  the  resistance,  the  more  marked  the  effect.  With 
10  or  12  Grove's  cups  several  inches  of  fine  steel  wire  may 
be  fused ;  and  with  a  powerful  battery,  several  yards  of 
platinum  wire  made  glowing  hot.* 

In  closing  or  breaking  the  circuit  we  produce  a  spark, 
the  size  of  which  depends  on  the  intensity  of  the  current. 
With  several  cells,  beautifully  variegated  sparks  are  ob- 
tained by  fastening  one 
pole  to  a  file  and  rub- 
bing the  other  upon  it. 
When  charcoal  or  gas- 
carbon  electrodes  are 
used  with  a  powerful 
battery,  on  slightly  sepa- 
rating the  points,  the 
intervening  space  is 
spanned  by  an  arch  of 
the  most  dazzling  light 
(Fig.  217).  The  flame, 
reaching  out  from  the 
positive  pole  like  a 
tongue,  vibrates  around 
the  negative  pole,  lick- 
ing now  on  this  side  and 

now  on  that.  The  heat  is  intense.  Platinum  melts  in  it 
like  wax  in  the  flame  of  a  candle,  \  the  metals  burn  with 
their  characteristic  colors  ;  and  lime,  quartz,  etc.,  are  fused. 

*  Torpedoes  and  blasts  are  fired  on  this  principle.  Two  copper  wires  leading 
from  the  battery  to  the  spot  are  separated  in  the  powder  by  a  short  piece  of  small 
steel-wire.  When  the  circuit  i.3  completed,  the  fine  wire  becomes  red-hot  and 
explodes  the  charge. 

t  To  show  the  varying  conducting  power  of  the  different  metals,  fasten  together 
alternate  lengths  of  silver  and  platinum  wire  and  pass  the  current  through  them. 
The  latter  will  glow  while  the  former  conveying  the  electricity  more  perfectly  will 
scarcely  manifest  ite  presence.  (See  also  p.  253.) 


VOLTAIC    ELECTRICITY.  237 

The  effect  is  not  produced  by  burning  the  charcoal  points, 
since  in  a  vacuum  it  is  equally  brilliant. 

(2.)  CHEMICAL  EFFECTS. — Electrolysis  (to  loosen  by  elec- 
tricity) is  the  process  of  the  decomposition  of  compound 
bodies  by  the  voltaic  current.  If  platinum  electrodes  be 
held  a  little  distance  apart  in  a  cup  of  water,  tiny  bubbles 

FIG.  218. 
H    o 


will  immediately  begin  to  rise  to  the  surface.  When  the  gases 
are  collected,  they  are  found  to  be  oxygen  and  hydrogen,  in 
the  proportion  of  two  parts  of  the  latter  to  one  of  the 
former.*  In  the  electrolysis  of  compounds,  their  elements 
are  found  to  be  in  different  electrical  conditions.  Hydrogen 
and  most  of  the  metals  go  to  the  negative  pole,  and  (since 
unlike  electricities  attract)  are  electro-positive.  Oxygen, 
chlorine,  sulphur,  etc.,  go  to  the  positive  pole,  and  are 
therefore  electro-negative. 

Electrotyping  is  the  process  of  depositing  metals  from 
their  solution  by  electricity.  It  is  used  in  copying  medals, 
woodcuts,  types,  etc.  An  impression  of  the  object  is  taken 

*  If  the  copper  poles  be  Inserted,  bubbles  will  pass  off  from  the  negative,  but 
none  from  the  positive  pole,  since  the  oxygen  combines  with  the  copper  wire. 
That  gas  has  no  effect  on  platinum.  The  burning  of  an  atom  of  zinc  in  the 
battery  develops  enough  electricity  to  set  free  an  atom  of  oxygen  at  the  positive 
pole.  It  is  interesting  to  notice  that  in  the  battery  there  is  zinc  burning,  i.  e.,  com- 
bining with  oxygen,  but  without  light ;  in  the  electric  light  the  real  force  of  the  com- 
bustion is  revealed.  We  may  thus  transfer  the  light  and  heat  to  a  great  distance 
(See  also  p.  254.) 


238  ELECTRICITY. 

with  gutta-percha  or  wax.  The  surface  to  be  copied  is 
brushed  with  black-lead  to  render  it  a  conductor.  The 
mould  is  then  suspended  in  a  solution  of  copper  sulphate, 
from  the  negative  pole  of  the  battery,  and  a  plate  of  copper 
is  hung  opposite  on  the  positive  pole.  The  electric  current 

FIG.  219. 


decomposes  the  copper  sulphate ;  the  metal  goes  to  the 
negative  pole  and  is  deposited  upon  the  mould,  while  the 
acid,  passing  to  the  positive  pole,  dissolves  the  copper,  and 
preserves  the  strength  of  the  solution.* 

Electro-plating  is  the  process  of  coating  with  silver  or  gold 
by  electricity.  The  metal  is  readily  deposited  on  German 
silver,  brass,  copper,  or  nickel  silver  (a  mixture  of  copper, 
zinc,  and  nickel).  The  objects  to  be  plated  are  thoroughly 
cleansed,  and  then  hung  from  the  negative  pole  in  a  solution 

*  While  the  plate  is  hanging  in  the  solution  there  is  no  noise  heard  or  bubbling 
seen.  The  most  delicate  sense  fails  to  detect  any  movement.  Yet  the  mysterious 
electric  force  is  continually  drawing  particles  of  ruddy,  solid  copper  out  of  the  blue 
liquid,  and,  noiselessly  as  the  fall  of  snowflakes,  dropping  them  on  the  mould ; 
producing  a  metal  purer  than  any  chemist  can  manufacture,  spreading  it  with  a 
uniformity  no  artist  can  attain,  and  copying  every  line  with  a  fidelity  that  knows  no 
mistake. 


YOLTAIC    ELECTRICITY.  239 

of  silver,  while  a  plate  of  silver  is  suspended  on  the  positive 
pole.  In  five  minutes  a  "  blush  "  of  the  metal  will  be  de- 
posited, which  conceals  the  baser  metal  and  is  susceptible 
of  polish.* 

(3.)  PHYSIOLOGICAL  EFFECTS. — With  a  single  cell  no 
effect  is  experienced  when  the  two  poles  are  held  in  the 
hands.  With  a  large  battery  a  sudden  twinge  is  felt,  and 
the  shock  becomes  painful  and  even  dangerous,  especially  if 
the  palms  are  moistened  with  water  to  increase  the  con- 
duction. Eabbits  which  had  been  suffocated  for  half  an 
hour,  have  been  restored  by  a  series  of  electric  shocks. 


4.    ELECTRO-MAGNETISM. 

1.  Effect  of  a  Voltaic  Current  on  a  Magnetic 
Needle. — If  a  current  of  electricity  be  passed  over  a  mag- 
netic needle,  the  needle  will  tend  to  place  itself  at  right 
angles  to  the  wire.  If  the  wire  is  brought  over  and  beneath 
the  needle,  it  doubles  the  effect,  and  the  play  of  the  needle 

*  Place  in  a  large  test-tube  a  silver  coin  with  a  little  nitric  acid.  If  the  fumes  of 
the  decomposed  acid  do  not  soon  rise,  warm  the  liquid.  When  the  silver  is  dissolved 
fill  the  tube  nearly  full  of  soft  water.  Next  drop  muriatic  acid  into  the  liquid  until 
the  white  precipitate  (silver  chloride)  ceases  to  fall.  When  the  chloride  has  settled, 
pour  off  the  colored  water  which  floats  on  top.  Fill  the  tube  again  with  soft  water ; 
shake  it  thoroughly  ;  let  it  settle,  and  then  pour  off  as  before.  Continue  this  process 
until  the  liquid  loses  all  color,  Finally,  fill  with  water  and  heat  moderately,  adding 
potassium  cyanide  (the  pupil  will  remember  that  this  substance  is  exceedingly  poi- 
sonous) in  small  bits  as  it  dissolves,  until  the  chloride  is  nearly  taken  up.  The  liquid 
is  then  ready  for  electro-plating.  Thoroughly  cleanse  a  brass  key,  hang  it  from  the 
negative  pole  of  a  small  battery  and  suspend  a  silver  coin  from  the  positive  pole. 
Place  these  in  the  silver  solution,  very  near  and  facing  each  other.  When  well 
whitened  by  the  deposit  of  silver  remove  the  key  and  polish  it  with  chalk.  In  the 
arts  the  polishing  is  performed  by  rubbing  with  "burnishers."  These  are  made  of 
polished  steel,  and  fit  the  surfaces  of  the  various  articles  upon  which  they  are  to  be 
used.  It  is  said  that  an  ounce  of  silver  can  be  spread  over  two  acres  of  surface.  A 
well-plated  spoon  receives  about  as  much  silver  as  there  is  in  a  ten-cent  piece.  The 
only  method  of  deciding  accurately  the  amount  deposited  is  by  weighing  the  article 
before  and  after  it  is  plated.— A  vessel  may  be  "  gold-lined  "  by  filling  it  with  a  solu- 
tion of  gold,  suspending  in  it  a  slip  of  gold  from  the  positive  pole  of  the  battery, 
and  then  attaching  the  negative  pole  to  the  vessel.  The  current  passing  through  the 
liquid  causes  it  to  bubble  like  soda-water,  and  in  a  few  moments  deposits  a  thin  film 
of  gold  over  the  entire  surface. 


240 


ELECTRICITY. 


becomes  a  delicate  test  of  the  presence  and  direction  of  the 
electric  force. 


m 


2.  The  Galvanometer  is  an  instrument  for  measuring 
the  force  and  direction  of  an  electric  current.  B  is  a  coil 
of  wire,  wound  with  thread  to  in- 
sulate it  and  compel  the  electricity 
to  pass  through  the  whole  length, 
entering  at  n  and  leaving  at  m. 


FIG.  221. 


ELECTRO-MAGNETISM. 


241 


The  silk  cord,  s,  supports  an  astatic  needle.  This  consists 
of  two  magnetic  needles,  one  over  the  graduated  circle  and 
the  other  within  the  coil,  with  the  north  pole  of  the  one 
opposite  the  south  pole  of  the  other,  so  as  to  neutralize  the 
attraction  of  the  earth,  and  permit  the  combined  needle  to  obey 
the  will  of  the  current  alone. 

FIG.  222. 

3.    Electro-magnets. — 

The  voltaic  current  produces 
magnetism.  If  a  current  be 
passed  through  the  wire  shown 
in  Fig.  211,  the  steel  bar  will 
be  rendered  magnetic.  This 
shows  the  identity  of  the  elec- 
tricity from  the  voltaic  bat- 
tery with  that  from  the  Leyden 
jar.  If  the  wire  be  wound 
around  a  bar  of  soft  iron,  as  in 
Fig.  222,  the  iron  will  instantly 

become  a  magnet  which  will  grasp  the 
armature  with  great  force,  but  will  as 
quickly  lose  its  properties  when  the  cur- 
rent is  broken.  If  the  current  be  passed 
through  a  coil  of  insulated  wire  (helix) 
(Fig.  223),  a  rod  of  iron  held  below  it 
will  be  drawn  up  forcibly,  as  if  pulled  by 
a  powerful  spring.*  Here  we  see  that  not 
only  does  the  soft  iron  within  become 
magnetic,  but  also  the  coil  itself. 

4.  Motion  Produced  by  Electricity. 
— If  we  reverse  the  direction  of  the  cur- 
rent, we  change  the  poles  of  the  magnet. 
Advantage  is  taken  of  this  to  produce 
continuous  motion.  Fig.  224  represents 
Page's  Rotating  Machine.  It  consists  of 

*  Thus  is  realized  in  science  the  fabulous  story  of  Ma- 
homet's coffin,  which  is  said  to  have  been  suspended  :n 
mid-air. 


FIG.  223. 


243  BLECTEICITY. 

an  upright  horse-shoe  magnet,  between  the  poles  of  which 
is  a  small  electro-magnet.      Above  this  are  two  springs, 
FIG  224  which  are  so  placed  that,  as  the  cen- 

tral rod  revolves  with  the  electro- 
magnet, the  current  passes  through 
these  springs,  alternately,  to  the  wire 
coiled  about  the  iron  of  the  electro- 
magnet. The  poles  of  the  electro- 
magnet are  thus  changed  twice  with 
each  revolution.  The  poles  of  the 
upright  magnet  attract  the  opposite 
poles  of  the  electro-magnet,  but  as 
soon  as  they  face  each  other  the  cur- 
rent is  reversed,  and  they  at  once 
repel  each  other  ;  the  other  poles  are 
then  attracted,  but  as  they  come 
together  are  repelled  as  before.  A  rapid  motion  is  thus 
secured.  The  revolutions  may  rise  as  high  as  2,500,  making 
5,000  reversals  of  the  current  in  a  minute. 

ELECTRO-MAGNETIC  ENGINES  are  constructed  either  on 
the  principle  that  the  magnet  retains  its  power  only  while 
the  current  is  passing,  or  that  the  poles  are  changed  by 
reversing  the  current.  They  have  been  made  of  8  or  10 
horse-power,  but  have  never  become  of  great  practical  value, 
because  of  the  expense  of  the  battery  required  to  produce 
the  electricity.  The  zinc  which  burns  in  the  cell  of  the 
electric-engine  is  far  more  expensive  than  the  coal  which 
burns  in  the  furnace  of  the  steam-engine. 

The  ELECTRO-MAGNETIC  TELEGRAPH  depends  on  the 
principle  of  closing  and  breaking  the  circuit  at  one  station, 
and  thereby  making  and  unmaking  an  electro-magnet  at  the 
station  to  which  the  despatch  is  to  be  sent.  A  single  wire 
is  used  to  connect  the  two  stations.  The  extremities  of  the 
wire  extend  into  the  ground,  and  the  earth  completes  the 
sircuit.  Each  station  has  a  key  and  a  register  (or  sounder) ; 


ELECTBO-MAGKETISM.  243 

the  former  is  used  for  sending  messages,  and  the  latter  for 
receiving  them. 

The  key  is  shown  in  Fig.  225.  E  and  F  are  screws  which 
fasten  the  instrument  to  the  table,  and  also  hold  the  two 
ends  of  the  wire.  F  is  insulated  by  a  rubber  ring  where  it 
passes  through  the  table  B.  H  is  a  lever  with  a  finger-button 
G,  a  spring  I,  to  keep  it  lifted,  and  a  screw  D,  to  regulate 
the  distance  it  can  move.  At  A  is  a  break  between  two 
platinum  points,  which  form  the  real  ends  of  the  wire. 

Fie.  825. 


When  G  is  depressed,  the  circuit  is  complete,  and  when 
lifted,  it  is  broken.  0  is  a  circuit-closer  that  is  used  when 
the  key  is  not  in  operation  ;  the  arm  being  pushed  under  A 
touches  the  platinum  wire  and  so  completes  the  circuit. 
The  operator,  by  moving  G-,  can  "close"  or  "open"  the 
circuit  at  pleasure.  He  thus  sends  a  message. 

The  register  contains  an  electro-magnet,  E  (Fig.  226). 
When  the  circuit  is  complete,  the  current,  passing  through 
the  coils  of  wire  at  E,  attracts  the  armature  77?.  This  ele- 
vates n,  the  other  end  of  tne  lever  mn,  and  forces  the  sharp 
point  x  firmly  against  the  soft  paper  a.  As  soon  as  the 


244 


ELECTRICITY. 


circuit  is  broken,  E  ceases  to  be  a  magnet,  and  the  spring 
E  lifts  the  armature,  drawing  the  point  from  the  paper. 
Clock-work  attached  to  the  rollers  at  z  moves  the  paper 
along  uniformly  beneath  the  point  x.  When  the  circuit  is 


FIG.  228. 


completed  and  broken  again  instantly,  there  is  a  sharp  dot 
made  on  the  paper.  This  is  called  e ;  two  dots,  * ;  three 
dots,  s ;  four  dots,  h.  If  the  current  is  closed  for  a  longer 
time,  the  mark  becomes  a  dash,  t ;  two  dashes,  m  ;  a  dot  and 
a  dash,  a. 

TABLE   OF   MORSE'S   SIGNS. 


0      ,     

j    

s  •  •  • 

b  —  .  ... 

k  

t  — 

c  .  .    . 

1   

u  •  •  — 

d  —  • 

m  

v  .  .  

e  . 

n  

w-  

f  
g  —  —  . 

o  •    • 

x  .  —  . 

7-    - 

h  

q  

Z    '  '  '      • 

i  -. 

r  •    •  • 

&.  ... 

A  skilful  operator  becomes  so  used  to  the  sound  that  the 
clicking  of  the  armature  is  perfectly  intelligible.  He  uses, 
therefore,  simply  a  "sounder"  i.e.,  a  register  without  the 
paper  and  clock-work  attachment. 


r  1 


BLEOTRO-MAQITETISM.  '  245 

RELAY.  —  When  the  stations  are  more  than  fifty  miles 
apart,  the  current  becomes  too  weak  to  work  the  register. 
The  relay  uses  the  force  of  a  local  battery  for  this  purpose. 


PIG.  227. 


D  is  the  line-wire  ;  0  the  ground-wire  ;  A  is  connected  with 
the  positive  pole ;  and  B  with  the  register  or  sounder  and 
thence  with  the  negative  pole  of  the  battery.  The  current 
passes  in  at  D,  traverses  the  fine  wire  of  the  electro-magnet, 
K,  and  thence  passes  out  at  0  to  the  ground.  The  arma- 
ture E,  playing  to  and  fro  as  the  current  from  the  distant 
station  darts  through  or  is  cut  off,  moves  the  lever  E.  This 
works  on  an  axis  at  the  lower  end  and  is  drawn  back  by  the 
spring  H,  which  is  regulated  by  the  thumb-screw  I.  As  E 
is  attracted  the  circuit  at  G  is  closed ;  the  current  from  A 
leaps  through  a  wire  underneath,  up  F,  and  down  L  and 
back  through  another  wire  underneath  to  B,  and  thence  to 
the  electro-magnet  of  the  register,  and  attracts  its  armature. 
The  operator  who  sends  the  message  simply  completes 
and  breaks  the  circuit  with  the  key ;  the  armature  of  the 
relay,  at  the  station  where  the  message  is  received,  vibrates 
in  unison  with  these  movements ;  the  register  or  sounder 
^epeats  them  with  greater  force ;  and  the  second  operator 
interprets  their  meaning. 

5.    Magneto-electricity    is    developed    by    means    of 
magnetism.     A  machine  for  this  purpose  is  shown  in  Eig. 


246 


ELECTRICITY. 


228.  Coils  of  wire  are  insulated  and  wound  around  a  small 
bar  of  soft  iron,  B,  bent  at  right  angles.  This  acts  as  the 
armature  of  a  powerful  horse-shoe  magnet,  before  the  poles 
of  which  it  revolves.  The  soft  iron  becomes  magnetic,  and 
then  induces  electric  currents  in  the  coils.  The  poles  are 

PIG.  228. 


changed  twice,  and  thus  two  opposite  currents  are  induced 
in  each  revolution.  By  means  of  a  break-piece  the  circuit 
is  rapidly  broken  and  closed.  Severe  shocks  are  thus  pro- 
duced, when  the  poles  are  grasped  by  the  hands.  * 

6.  Induced  Currents. — Let  two  coils  of  wire  be  made 
to  fit  into  each  other,  and  carefully  separated  by  insulators. 
If  a  current  of  electricity  be  passed  through  the  inner  coil,  it 
will  induce  a  powerful  secondary  current,  flowing  in  the 

*  In  Wild's  machine,  the  induced  current  from  the  coils  acts  upon  a  large  electro- 
magnet,  which  is  thereby  excited  to  a  high  degree.  A  machine  driven  by  a  steam- 
engine  of  15-horse  power  produces  an  electric  light  dazzling  as  the  noonday  sun, 
throwing  the  flame  of  the  street-lamps  into  shade  at  a  long  distance.  Its  heat  is  suf- 
ficient to  fuse  a  -i-inch  bar  of  iron  fifteen  inches  long  or  7  foot  of  No.  6  iron  wire. — 
"  A  Yankee  once  threw  the  industrial  world  of  Europe  into  a  wonderful  excitement  by 
announcing  a  new  theory  of  perpetual  motion  based  on  the  magneto-electric  machine. 
He  proposed  to  decompose  water  by  the  current  of  electricity  ;  then  burn  the  hydro 
gen  and  oxygen  thus  obtained.  In  this  way  he  would  drive  a  small  steam-engine, 
which,  in  turn,  would  keep  the  magneto-electric  machine  in  motion.  This  would 
certainly  be  a  splendid  discovery.  It  would  be  a  steam-engine  which  would  prepare 
its  own  fuel,  and,  in  addition,  dispense  light  and  heat  to  all  around."  (llclmholtz.) 
(See  also  p.  2&4.) 


ELECTKO-MAGtfETISM.  247 

opposite  direction,  in  the  outer  coil.     This  soon  ceases  ;  on 
breaking  the  circuit,  however,  it  will  start  again,  but  in  the 
same  direction  as  the 
primary  current.     The 
apparatus    shown     in 
Fig.  229  consists  essen- 
tially of  the  two 
coils   just  de- 
scribed.     The 
primary     cur- 
rent    from     a 
single    cell    is 

rapidly  interrupted  by  means  of  a  small  electro-magnet. 
When  this  is  magnetized,  it  attracts  the  armature,  and  thus 
the  circuit  is  broken ;  the  armature  immediately  springs 
back,  and  again  completes  the  circuit.  A  bunch  of  iron  wires 
may  be  inserted  as  a  core  in  the  inner  coil.  When  the 
current  passes,  these  become  magnetized,  and  by  induction 
strengthen  the  secondary  current.  Ruhmlcorff's  coil  is  con- 
structed on  the  same  principle.  The  largest  coils  often 
contain  thirty  to  fifty  miles  of  covered  wire.  They  will 
throw  a  quick  succession  of  sparks,  20  to  40  inches  long,  so 
as  to  charge  and  discharge  a  Ley  den  jar,  with  a  crack  like 
that  of  a  pistol,  as  rapidly  as  one  can  count.* 

7.  The  Telephone!  (sound  afar),  in  the  Bell  instru- 
ment, consists  of  a  magnet  N,  S,  at  one  end  of  which  is  a 
coil  of  wire  0,  a  thin  iron  plate  B,  and  a  mouth-piece  A. 

*  They  are  also  used  to  illustrate  the  effects  of  the  passage  of  electricity  through 
the  various  gases  and  rarefied  vapors.  These  are  placed  in  sealed  glass  tubes,  known 
as  Geiesler's  tubes,  and  when  the  current  passes  they  exhibit  the  richest  tints  and 
bands  of  color. 

t  A  telephone  in  parts,  ready  to  be  put  together  by  the  experimenter,  is  sold  by 
the  apparatus  dealers.  A  simple  but  effective  instrument  can  be  made,  at  a  slight 
expense,  by  a  pupil  with  ordinary  mechanical  ability.  One  process,  with  illustrative 
drawings,  is  given  in  the  Popular  Science  Monthly,  March,  1878,  and  another  in  the 
Scientific  American,  Vol.  39,  No.  5.  In  Vol.  39,  No.  16,  is  also  described  a  method  of 
constructing  a  microphone  ;  and  in  the  Scientific  American  Supplement,  No.  133,  is 
an  account  of  a  home-made  phonograph.  These  numbers  can  be  procured  by  any 
newsdealer.  In  using  the  telephone,  two  instruments  exactly  alike  are  employed. 
One  is  ueld  to  the  mouth  of  the  speaker,  and  the  other  to  the  ear  of  the  listener 


248 


ELECTRICITY. 


The  ends  of  the  coil  are  connected,  the  one  with  a  line  wire 
and  the  other  with  the  earth  or  a  return  wire.     At  the 

second  station 
is  a  similar  in- 
strument. The 
vibrations  of  the 
voice  at  A  throw 
the  plate  into 
vibration.  As 
B  approaches  N 
the  magnet  is 
strengthened 
and  a  current  of 
electricity  in- 
duced in  the 
coil  0  and 
When  B  recedes  from 


thence  transmitted  to  the  line  wire. 
N  the  current  is  stopped.  Thus 
the  waves  of  air  are  translated 
into  corresponding  waves  of  elec- 
tricity.* At  the  other  station 
the  reverse  process  occurs.  The 
pulses  of  electricity  are  there 
converted  into  waves  of  air 
which,  falling  upon  the  ear  at 
A,  produce  the  phenomena  of 
sound. 

8.  The  Microphone  f  con- 
sists of  a  -small  battery  for 
generating  a  weak  current  of 
electricity,  a  telephone  for  the 


Fia. 


*  It  should  be  noticed  that  in  the  apparatus  described  in  the  note  on  p.  127,  the 
sound  is  conveyed  by  mechanical  vibrations  merely,  while  in  the  true  telephone  they 
are  transmitted  by  electrical  pulsations. 

t  The  microphone  is  the  last  refinement  of  the  telegraph.  The  faintest  shake  of 
the  carbon  rod  produces  a  minute  breaking  of  contact  or  increase  of  resistance  to  the 
electric  current.  This  produces  a  corresponding  change  of  magnetic  strength  in  t!ie 
telephone. 


THERMAL    ELECTRICITY.  249 

receiving  instrument,  and  a  transmitting  or  speaking  instru- 
ment. The  last  may  be  a  small  rod  of  gas  carbon  (Fig. 
231),  with  the  ends  set  loosely  in  blocks  of  the  same  mate- 
rial ;  the  latter  are  attached  to  an  upright  support  glued 
into  a  wooden  baseboard.  This  instrument  is  connected 
with  the  battery  and  the  telephone.  A  sound  made  near 
the  rod  can  be  heard  at  the  distant  telephone,  with  wonder- 
ful distinctness.  The  patter  of  a  fly's  foot  in  walking  across 
the  baseboard,  or  the  brush  of  a  camel's-hair  pencil,  is 
audible  many  miles  away. 

5.    THERMAL    ELECTRICITY. 

As  electricity  can  be  changed  into  heat,  heat  can  in  turn 
be  converted  into  electricity. 

A  Thermo-electric  Pile  consists  of  alternate  bars  of 
antimony  and  bismuth  soldered  together,  as  shown  in 
Fig.  233.  When  mounted 
for  use,  the  couples  are 
insulated  from  each  other 
and  enclosed  in  a  copper 
frame  P.  If  both  faces  of 
the  pile  are  equally  heated, 
there  is  no  current.  The 
least  variation  of  temper- 
ature, however,  between 
the  two  is  indicated  by  the 
flow  of  electricity.  Wires  from  «,  the  positive  pole,  and  b, 
the  negative,  connect  the  pile  with  the  galvanometer  (Fig. 
221).  This  constitutes  a  delicate  test  of  the  presence  of 
heat.  A  fly  walking  over  the  face  of  the  pile  by  its  warmth 
will  move  the  needle,  if  the  galvanometer  be  very  delicate.* 


*  Strange,  that  minute  quantities  of  heat  become  sensible  only  when  they  are 
converted  into  electricity,  then  into  magnetism,  and  lastly  into  motion. 
(See  also  p.  254.) 


250  ELECTRICITY. 

6.    ANIMAL    ELECTRICITY. 

Electric  Fish  have  the  property  of  giving,  when 
touched,  a  shock  like  that  from  a  Leyden  jar.  The  torpedo 
and  the  electrical  eel  are  most  noted.  The  former  is  a 
native  of  the  Mediterranean,  and  its  shock  was  anciently 
prized  as  a  cure  for  various  diseases.  The  latter  is  abundant 
in  certain  South  American  waters.  A  specimen  of  this  fish, 
forty  inches  in  length,  was  estimated  by  Faraday  to  emit  a 
spark  equal  to  the  discharge  of  a  battery  of  fifteen  Leyden 
jars. 


SUMMARY. 

Electricitj  is  a  form  of  energy  which  is  excited  by  means  of  mag- 
netism, friction,  chemical  action,  heat,  and  also  by  induction  or  the 
presence  of  other  electrified  bodies.  It  possesses  a  peculiar  duality  or 
doubleness  of  property,  as  seen  in  the  alternate  attraction  and  repul- 
sion of  magnetic  poles.  These  two  forms  of  the  same  force  are  called 
positive  and  negative.  Magnetism  is  manifested  only  at  the  ends  of 
a  body,  while  electricity  may  reside  equally  over  its  entire  surface. 
Magnetism  is,  however,  identical  with  electricity,  and  they  have  a  re- 
ciprocal relation.  Magnets  of  soft  iron  may  be  made  of  any  strength 
by  means  of  the  electrical  currents.  The  earth  itself  is  doubtless  a 
great  magnet,  made  so  by  electrical  currents  which  circulate  about  it, 
being  generated  perhaps  largely  by  the  heat  of  the  sun.  This  accounts 
for  the  polarity  of  the  needle  which  is  so  useful  in  navigation.  Elec- 
tricity may  be  developed  in  any  quantity  by  means  of  friction,  com- 
bined, as  in  Holtz's  machine,  with  induction.  It  may  be  hoarded  for 
use  by  the  Leyden  jar,  the  construction  of  which  curiously  illustrates 
the  phenomena  of  lightning. 

Galvanic  electricity  is  born  of  metallic  difference  and  chemical 
action.  The  essentials  of  an  ordinary  battery  for  its  development  are 
two  substances,  one  of  which  is  more  strongly  acted  on  by  a  chemical 
agent,  while  the  other  gathers  the  electricity  set  free.  It  differs  from 
frictional  electricity  in  being  characterized  by  quantity  rather  than 
intensity;  in  flowing  in  a  continuous  current  rather  than  a  sudden 
discharge  ;  in  traveling  along  a  wire  to  any  desired  distance ;  in  being 


HISTORICAL    SKETCH.  251 

perfectly  manageable,  and  in  manifesting  its  properties  at  any  point 
where  the  current  is  interrupted.  It  has  a  powerful  chemical  or  inter- 
atomic action,  breaking  up  the  molecules  of  compound  bodies  as  seen 
in  the  decomposition  of  water  and  the  various  processes  of  electro- 
typing,  plating,  etc.  The  electric  telegraph,  the  telephone,  the  micro- 
phone and  the  use  of  the  electric  light  for  illumination,  are  the  latest 
applications  of  the  electric  force  to  the  purposes  of  practical  life. 


HISTORICAL    SKETCH. 

Thales  (6th  cent.  B.  C.),  one  of  the  eeven  wise  men,  knew  that 
when  amber  is  rubbed  with  silk  it  will  attract  light  bodies,  as  straw, 
leaves,  etc.  This  property  was  considered  so  marvellous  that  amber 
was  supposed  to  possess  a  soul.  From  the  Greek  name  of  the  sub- 
stance (elektron)  our  word  electricity  is  derived.  This  simple  phe- 
nomenon constituted  all  that  was  known  until  the  16th  century,  when 
Gilbert,  physician  to  Queen  Elizabeth,  made  some  valuable  experi- 
ments. The  next  century  Guericke  discovered  induction.  Then 
Newton  turned  his  powerful  mind  upon  the  subject  and  is  thought  by 
some  to  have  invented  a  glass-globe  electrical  machine.  In  the  18th 
century,  Du  Fay  accounted  for  electrical  phenomena  by  the  theory  of 
two  fluids — the  vitreous  and  the  resinous.  This  view  is  yet  held  by 
many  and  its  nomenclature  has  permeated  the  whole  subject  of  elec- 
tricity so  that  we  still  speak  of  a  "  current,"  a  "  flow  of  electricity,"  etc. 
About  1752,  Franklin  proved  the  identity  of  lightning  and  fric- 
tional  electricity  by  means  of  a  kite  made  of  a  silk  handkerchief  and 
with  a  pointed  wire  at  the  top.  He  elevated  this  during  a  thunder- 
storm, tying  at  the  end  of  the  hemp  string  a  key,  and  then  insulating 
the  whole  by  fastening  it  to  a  post  with  a  long  piece  of  silk  lace.  On 
presenting  his  knuckles  to  the  key,  he  obtained  a  spark.  So  great 
was  his  joy,  that  he  is  said  to  have  burst  into  tears.  He  afterwards 
charged  a  Leyden  jar,  and  performed  other  electrical  experiments  in 
this  way.  These  attempts  were  attended  with  very  great  danger. 
Prof.  Richman,  of  St.  Petersburg,  drew  in  this  manner  from  the  clouds 
a  ball  of  blue  fire  as  large  as  a  man's  fist  which  struck  him  lifeless. 

In  the  year  1790  Galvani  was  engaged  in  some  experiments  on 
animal  electricity.  For  this  purpose  he  used  frogs'  legs  as  electro- 
scopes. He  had  hung  several  of  these  upon  copper  hooks  from  the 
iron  railing  of  the  balcony,  in  order  to  see  what  effect  the  atmospheric 
electricity  might  have  upon  them.  He  noticed,  to  his  surprise, 
that  when  the  wind  blew  them  against  the  iron  supports,  the 
legs  were  convulsed  as  if  in  pain.  After  repeated  experiments, 


252  ELECTRICITY. 

Galvani  concluded  that  this  effect  was  produced  by  what 
he  termed  animal  electricity,  that  this  electricity  is  different 
from  that  caused  by  friction,  and  that  he  had  discovered 
the  agent  by  which  the  will  controls  the  muscles.  Volta 
rejected  the  idea  of  animal  electricity,  and  held  that  the 
contact  of  dissimilar  metals  was  the  source  of  the  elec- 
tricity, while  the  frog  was  "  only  a  moist  conductor,  and 
for  that  purpose  was  not  as  good  as  a  wet  rag."  He  ap- 
plied this  view  to  the  construction  of  "Volta's  pile," 
which  is  composed  of  plates  of  zinc  and  copper,  between 
which  are  laid  pieces  of  flannel  moistened  with  an  acid  or  a  saline 
solution  (Fig.  233).  This  theory  is  substantially  the  one  held  at  the 
present  time,  though  we  now  know  that  there  must  be  chemical 
action  to  continue  the  supply. 

Electricity  and  magnetism  were  studied  as  distinct  branches  until 
1820,  when  Oersted  of  Copenhagen  discovered  the  phenomenon  shown 
in  Fig.  220.  This  was  published  everywhere,  and  excited  the  deepest 
interest  of  scientific  men.  In  the  fruitful  mind  of  Ampere  the  experi- 
ment bore  abundant  fruit.  He  discovered  that  two  parallel  wires  con- 
veying electricity  in  the  same  direction  attract  each  other,  and  when 
in  opposite  directions,  repel  each  other.  From  this  he  generalized  the 
entire  subject.  Prof.  Henry  next  exhibited  the  wonderful  power  of 
the  electro-magnet,  and  invented  the  electro-magnetic  engine.  Scien- 
tific men  in  all  parts  of  the  world  were  now  gathering  the  material 
necessary  for  the  invention  of  the  electric  telegraph.  It  fell  to  Samuel 
F.  B.  Morse  to  make  this  knowledge  practical,  and  in  1837  he  exhibited 
in  New  York  a  working  instrument.  An  experimental  line  betweeu 
Washington  and  Baltimore  was  completed  in  1844,  and,  on  May  27tb 
of  that  year,  was  sent  the  first  message  ever  forwarded  by  a  recording 
telegraph. 

Consult  Maxwell's  "  Electricity  and  Magnetism  "  ;  Tyndall's  "  Les- 
sons in  Electricity  "  ;  Faraday's  "  Lectures  on  the  Physical  Forces  " 
and  "Kesearches  in  Electricity"  ;  Noad's  "Manual  of  Electricity"; 
Art.  on  the  Microphone,  in  Scribner's  Monthly,  Vol.  XVI,  p.  GOO  ; 
Prescott's  "The  Speaking  Telephone,  Talking  Phonograph?'  etc.; 
Foster's  "Electrical  Measurements,"  in  Science  Lectures  at  South 
Kensington,  Vol.  I,  p.  264  ;  Thomson's  "  Papers  on  Electrostatics  and 
Magnetism  "  ;  Guillemin's  "  The  Forces  of  Nature  "  and  "  The  Appli- 
cations of  Physical  Forces";  "American  Cyclopaedia,"  Articles  on 
Electricity,  Magnetism,  Electro-magnetism,  etc.;  Smith's  "Manual  of 
Telegraphy  "  ;  Jones's  "  Historical  Sketch  of  Electric  Telegraph  "  ; 
Watts's  "  Electro-metallurgy  " ;  "  Barnes's  Hundred  Years  of  American 
Independence,"  Sec.  on  Morse,  p.  442 ;  "  Fourteen  Weeks  in  Zoology," 
Sec.  on  Torpedo,  p.  186 ;  Gordon's  "  Electricity  and  Magnetism  " ;  Hos- 
pitaller's "  Modern  Applications  of  Electricity  "  (specially  commended 
for  latest  discoveries). 


ADDITIONAL  FACTS  ABOUT  ELECTRICITY. 


(P.  234.)  In  Smee's  'Battery,  the  "  polarization  of  the  plates  "  due, 
among  other  causes,  to  the  adherence  of  the  hydrogen  to  the  copper, 
and  hence  the  production  of  a  reverse  electro-motive  force,  is  partially 
prevented  by  the  roughening  of  the  silver  plate  ;  in  Grove's  Battery, 
the  polarization  is  remedied  by  the  nitric  acid  which  intercepts  the 
hydrogen ;  in  the  Leclanche  Battery,  the  depolarization  is  effected  by 
means  of  manganese  dioxide  ;  and  in  the  famous  battery  of  13,000  ele- 
ments, used  by  Warren  de  la  Rue,  silver  chloride  is  employed  as 
depolarizer.  In  some  cases,  the  depolarizing  and  the  exciting  agent 
both  form  a  single  solution  ;  as,  for  example,  in  Grenet's  battery, 
where  the  zinc  arid  the  carbon  are  immersed  in  a  solution  of  sulphuric 
acid  and  potassium  bichromate. 

(P.  235.)  The  electrical  units  now  generally  recognized  are  based 
upon  the  Centimetre,  the  Gramme,  and  the  Second  (C.  G.  S.).  i.  The 
ohm  is  the  unit  of  the  resistance  of  the  conductor  through  which  the 
current  passes  ;  it  corresponds  to  the  resistance  of  an  iron  wire  four 
millimetres  in  diameter,  and  a  hundred  metres  long.  The  resistance 
of  a  Daniell's  cell  is  about  half  an  ohm,  and  of  250  feet  of  No.  16 
ordinary  copper  wire,  about  an  ohm.  In  general,  the  resistance  of 
wires  of  the  same  size  and  material  is  inversely  as  their  cross-section. 
2.  The  volt  is  the  unit  of  the  electro-motive  force  (E.  M.  F.) ;  it  cor- 
responds very  nearly  to  the  E.  M.  F.  (difference  of  potential)  generated 
by  one  Daniell's  cell.  3.  The  ampere  is  the  unit  of  intensity ;  it  repre- 
sents the  current  produced  by  "  a  volt  in  an  ohm."  According  to  Ohm's 

law,  THE  INTENSITY  IS  EQUAL  TO  ELECTRO -MOTIVE  FORCE  DIVIDED  BY  THE 

RESISTANCE."  4.  The  coulomb  is  the  unit  of  quantity  ;  it  represents  the 
quantity  of  electricity  yielded  in  one  second  by  a  current  with  an  inten- 
sity of  one  ampere.  5.  The  farad  is  the  unit  of  capacity;  it  represents 
the  capacity  of  a  condenser  containing  a  coulomb  when  charged  to  the 
potential  of  a  volt. 

(P.  236.)     There  are  two  forms  of  the  electric  light  now  used  — the 


254  ELECTRICITY. 

arc  (shown  in  Fig.  217),  where  the  current  passes  between  two  carbon 
points  ;  and  the  incandescent,  where  the  current  heats  to  a  dazzling  white 
a  carbon  strip  placed  in  the  circuit.  The  former,  as  in  the  Brush  sys- 
tem, is  employed  in  lighting  streets,  railroad  stations,  and  large  halls  ; 
the  latter  is  generally  used  in  dwellings,  etc.,  as  it  gives  a  softer  light, 
and  is  much  more  steady.  When  exposed  to  the  air,  the  voltaic  arc 
rapidly  wastes  the  carbon  points.  Electric  lamps  have  therefore  been 
devised  that,  by  a  self-acting  apparatus,  keep  the  points  at  a  proper 
distance  from  each  other.  In  the  Jablochkoff  Candle  two  carbon  rods 
are  placed  side  by  side,  and  are  insulated  from  each  other  by  kaolin  ; 
when  an  electric  current  is  passed  through  them,  the  arc  is  produced 
at  the  upper  end  of  the  rods,  and  the  clay  wastes  with  the  carbon. 
Edison's  Lamp  consists  of  a  tiny  carbon  loop  placed  in  a  glass  globe 
from  which  the  air  has  been  so  completely  exhausted  as  to  leave  only 
i^oopoo  of  an  atmosphere. 

(P.  237.)  Any  battery-cell  subjected  to  electrolysis  is  called  a 
secondary  cell.  Several  such  cells  form  an  accumulator  or  storage 
battery.  Faure's  Accumulator  consists  of  two  lead  plates  coated  with 
red  lead,  rolled  together  with  flannel  between  them,  and  immersed  in 
dilute  sulphuric  acid.  A  current  of  electricity  passed  through  such  a 
cell  changes  a  part  of  the  red  lead  on  the  positive  plate  into  peroxide 
of  lead  ;  and  a  part  of  the  red  lead  on  the  negative  plate  into  spongy 
metallic  lead.  A  battery  of  these  cells  when  freshly  charged  will  retain 
its  electricity  and  produce  a  sustained  current  when  desired. 

(P.  246.)  The  powerful  dynamo,  or,  more  fully,  dynamo-electric 
machines  driven  by  steam-power  and  used  for  electric  lighting,  etc.,  are 
based  upon  the  general  principle  of  the  magneto-electric  machine  shown 
in  Fig.  228.  The  fixed  magnets  (known  as  the  field  magnets)  are  now 
usually  powerful  electro-magnets  excited  to  intense  action  by  arma- 
tures revolving  at  a  high  speed.  For  a  full  account  and  pictures  of  the 
various  forms  of  dynamos,  read  Hospitalier's  Modern  Applications  of 
Electricity,  translated  by  Maier. 

(P.  249.)  Langley's  Bolometer  is  an  instrument  devised  for  measur- 
ing minute  degrees  of  heat.  It  will  detect  a  change  of  temperature  of 
.00001°  C.  The  apparatus  depends  on  the  principle  that  when  we 
warm  one  of  the  two  wires  from  a  battery,  making  the  arms  of  an 
"  electric  bridge,"  a  diminished  current  is  caused  by  the  heat,  and  the 
needle  of  a  galvanometer  may  thus  be  made  to  move. 


CONCLUSION. 

"Science  Is  a  psalm  and  a  prayer."— P, 


NOWHERE  in  nature  do  we  find  chance.  Every  event  is 
governed  by  fixed  laws.  If  we  would  accomplish  any  result 
or  perform  any  experiment,  we  must  come  into  exact  har- 
mony with  the  universal  system.  If  we  deviate  from  the 
line,  of  law  by  a  hair's  breadth,  we  fail.  These  laws  have 
been  in  operation  since  the  creation,  and  all  the  discoveries 
of  science  prove  them  to  extend  to  the  most  distant  star  in 
space.  A  child  of  to-day  amuses  itself  with  casting  a  stone 
into  the  brook  and  watching  the  widening  curves ;  little 
antediluvian  children  may  have  done  the  same.  A  law  of 
nature  has  no  force  of  itself  ;  it  is  but  the  manner  in  which 
force  acts. 

We  cannot  create  force.  We  can  only  take  it  as  a  gift 
from  God.  We  find  it  everywhere  in  Nature;  so  that 
matter  is  not  dumb,  but  full  of  inherent  energy.  A  tiny 
drop  of  dew  sparkling  on  a  spire  of  grass  is  instinct  with 
power  :  Gravity  draws  it  to  the  earth ;  Chemical  Affinity 
binds  together  the  atoms  of  hydrogen  and  oxygen  ;  Cohesion 
holds  the  molecules  of  water,  and  gathers  the  drop  into  a 
globe  ;  Heat  keeps  it  in  the  liquid  form  ;  Adhesion  causes 
it  to  cling  to  the  leaf.  If  the  water  be  decomposed,  Elec- 
tricity will  be  set  free ;  and  from  this,  Heat,  Light,  Mag- 
netism, and  Motion  can  be  produced.  Thus  the  commonest 
object  becomes  full  of  fascination  to  the  scientific  mind, 
since  in  it  reside  the  mysterious  forces  of  Nature. 

These  various  forces  can  be  classified  either  as  attractive 
or  repellent.  Under  their  influence  the  atoms  or  molecules 
resemble  little  magnets  with  positive  and  negative  poles. 
They  therefore  approach  or  recede  from  one  another,  and 
so  tend  to  arrange  themselves  according  to  some  definite 


256  OOKCLUSIOK. 

plan.  "  The  atoms  march  in  time,  moving  to  the  music  of 
law."  A  crystal  is  but  a  specimen  of  "molecular  archi- 
tecture" built  up  by  the  forces  with  which  matter  is 
endowed.  Forces  continually  ebb  and  flow,  but  the  sum  of 
energy  through  the  universe  remains  the  same.  In  time  all 
the  possible  changes  may  be  rung,  and  the  various  forms  of 
energy  subside  into  one  uniformly-diffused  heat-quiver,  but 
in  that  will  exist  the  representation  of  all  the  forces  which 
now  animate  creation,  and,  as  we  believe,  matter  and  force 
will  perish  only  together. 

The  forces  of  Nature  are  strangely  linked  with  our  lives. 
Everywhere  a  Divine  Hand  is  developing  ideas  tenderly  and 
wondrously  related  to  human  needs.  To  the  thoughtful 
mind  all  phenomena  have  a  hidden  meaning. 

"  To  matter  or  to  force 

The  all  is  not  confined  ; 

Beside  the  law  of  things 

Is  set  the  law  of  mind ; 

One  speaks  in  rock  and  star, 

And  one  within  the  brain, 

In  unison  at  times, 

And  then  apart  again. 
And  both  in  one  have  brought  us  hither, 
That  we  may  know  our  whence  and  whither. 

"  The  sequences  of  law 

We  learn  through  mind  alone ; 

We  see  but  outward  forms, 

The  soul  the  one  thing  known; 

If  she  speak  truth  at  all, 

The  voices  must  be  true 

That  give  these  visible  things, 

These  laws,  their  honor  due, 
But  tell  of  One  who  brought  us  hither, 
And  holds  the  keys  of  whence  and  whither. 

"  He  in  His  science  plans 

What  no  known  laws  foretell ; 

The  wandering  fires  and  fixed 

Alike  are  miracle  : 

The  common  death  of  all, 

The  life  renewed  above, 

Are  both  within  the  scheme 

Of  that  all-circling  love. 
The  seeming  chance  that  cast  us  hither 
Accomplishes  His  whence  and  whither." 


X. 
APPENDIX. 


QUESTIONS. 

THE  following  questions  are  those  which  the  author  has  used  in  his 
classes,  both  as  a  daily  review  and  for  examination.  A  standing 
question,  which  has  followed  every  other  question,  has  been :  "  Can 
you  illustrate  this  ?  "  Without,  therefore,  a  particular  request,  the  pupil 
has  been  accustomed  to  give  as  many  practical  examples  as  he  could, 
whenever  he  has  made  any  statement  or  given  any  definition.  The 
figures  refer  to  the  pages  of  the  book. 

I.  Introduction.— Define  matter.  A  body.  A  substance.  Name 
and  define  the  two  kinds  of  properties  which  belong  to  each  substance. 
State  the  suppositions  of  the  Atomic  Theory.  What  is  a  molecule  ? 
An  atom  ? 

14.  Describe  the  two  kinds  of  change  to  which  matter  may  be  sub- 
jected.    What  is  the  principal  distinction  between  Philosophy  (Physics) 
and  Chemistry?     Mention   some  phenomena  which  belong  to  each. 
Why  are  these  branches  intimately  related  ? 

15.  Name  the  general   properties   of   matter.      Define   magnitude. 
Size.     Distinguish  between  size  and  mass.*    (See  notes,  pp.  21,  53.) 
Why  are  feathers  light  and  lead  heavy?     Why  is  it  necessary  to  have  a 
standard  of  measure  ?     What  are  the  French  and  English  standards  ? 

*  "  In  a  rude  age,  before  the  invention  of  means  for  overcoming  friction,  the  weight 
of  bodies  formed  the  chief  obstacle  to  setting  them  in  motion.  I.  was  only  after 
some  progress  had  been  made  in  the  art  of  throwing  missiles,  and  in  the  use  of  wheel 
carriages  and  floating  vessels,  that  men's  minds  became  practically  impressed  with 
the  idea  of  mass  as  distinguished  from  weight.  Accordingly,  while  almost  all  the 
metaphysicians  who  discussed  the  qualities  of  matter,  assigned  a  prominent  place  to 
weight  among  the  primary  qualities,  few  or  none  of  them  perceived  that  the  sole 
unalterable  property  of  matter  is  its  mass.  At  the  revival  of  science  this  property 
was  expressed  by  the  phrase  '  The  inertia  of  matter  ; '  but  while  the  men  of  science 
understood  by  this  term  the  tendency  of  the  body  to  persevere  in  its  state  of  motion 
(or  rest),  and  considered  it  a  measurable  quantity,  those  philosophers  who  were 
unacquainted  with  science  understood  inertia  in  its  literal  sense  as  a  quality— mere 
want  of  activity  or  laziness.  I  therefore  recommend  to  the  student  that  he  should 
impress  his  mind  with  the  idea  of  mass  by  a  few  experiments,  such  as  setting  in 
motion  a  grindstone  or  a  well-balanced  wheel,  and  then  endeavoring  to  stop  it, 
twirling  a  long  pole,  etc.,  till  he  comes  to  associate  a  set  of  acts  and  sensations  with 
the  scientific  doctrines  of  dynamics,  and  he  will  never  afterward  be  in  any  danger 
of  loose  ideas  on  these  subjects."— Maxwelfs  Theory  of  Heat,  p.  85. 


258  QUfiSTIOKS. 

Give  the  history  of  the  English  standard.  (See  pp.  23,  24.)  Is  the 
American  yard  an  exact  copy  of  the  English  ?  Give  an  account  of  the 
French  system.  By  what  name  is  this  system  commonly  known?  Is 
either  of  these  systems  founded  on  a  natural  standard?  Why  is  it 
desirable  to  have  such  a  standard  ? 

16-17.  Define  Impenetrability.  Give  some  apparent  exceptions,  and 
explain  them.  Define  Divisibility.  Is  there  any  limit  to  the  divisi- 
bility of  matter  ?  Define  Porosity.  Is  the  word  porous  used  here  in 
its  common  acceptation  ?  Compare  the  size  of  an  atom  or  a  molecule 
with  that  of  a  pore.  What  practical  use  is  made  in  the  arts  of  the 
property  of  porosity?  Describe  the  experiment  of  the  Florence 
academicians. 

1 8.  Define  Inertia.     Does  a  ball,  when  thrown,  stop  itself?    Why  is 
it  difficult  to  start  a  heavy  wagon?    Why  is  it  dangerous  to  jump  from 
the  cars  when  in  motion?     (Compare  First  Law  of  Motion,  p.  28.) 
Define  Indestructibility.     Did  the  earth,  at  its  creation,  contain  the 
same  quantity  of  matter  that  it  does  now? 

19.  Name  the  specific  properties  of  matter.     Define  Ductility.     How 
is  iron  wire  made?     Platinum  wire?     Gilt  wire?     Define  Malle- 
ability. 

•  20.  Describe  the  manufacture  of  gold-leaf.  Is  copper  malleable  ? 
Define  Tenacity.  Name  and  define  the  three  kinds  of  Elasticity. 
Illustrate  the  elasticity  of  compression  as  seen  in  solids.  In  liquids. 
In  gases.  What  is  said  about  the  relative  compressibility  of  liquids 
and  gases  ? 

21.  Illustrate  the  elasticity  of  expansion  as  seen  in  solids,  liquids, 
and  gases.  Define  Elasticity  of  Torsion.  What  is  a  Torsion  balance  ? 
Define  Hardness.  Does  this  property  depend  on  density?  Define 
Density.  Define  Brittleness.  Is  a  hard  body  necessarily  brittle? 
Name  a  brittle  and  a  hard  body. 

II.  Motion  and  Force. — Define  motion,  absolute  and  relative. 
Rest.  Velocity.  Force.  What  are  the  resistances  to  motion?  Tell 
what  you  can  about  friction.  Why  does  oil  diminish  friction  ?  (See 
p.  39.)  What  uses  has  friction  ?  What  law  governs  the  resistance  of 
air  or  water  ?  Define  Momentum. 

28-30.  Show  that  motion  is  not  imparted  instantaneously.  State  the 
three  laws  of  motion  and  the  proof  of  each.  If  a  ball  be  fired  into  the 
air  when  a  horizontal  wind  is  blowing,  will  it  rise  as  high  as  if  the  air 
were  still  ?  Describe  the  experiments  with  the  collision  balls.  Give 
practical  illustrations  of  action  and  reaction.  If  a  bird  could  live, 
could  it  fly  in  a  vacuum  ?  Define  compound  motion. 

31.  Define  the  so  called  "  parallelogram  of  forces."  The  resultant. 
How  can  the  resultant  of  two  or  more  forces  be  found  ?  Name  some 


QUESTIONS.  259 

practical  illustrations  of  compound  motion.     What  is  the  "  resolution 

of  forces  ?  " 

32.  Show  how  one  vessel  can  sail  south  and  another  north,  driven 
by  the  same  westerly  wind.     Explain  how  a  kite  is  raised.     Explain 
the  "  split-shot"  in  croquet. 

33.  Explain  the  towing  of  a  canal-boat.    Describe  how  motion  in  a 
curve,  and  circular  motion  are  produced.     Explain  the  centripetal  and 
centrifugal  forces. 

34.  Show  when  the  centrifugal  force  becomes  strong  enough  to  over- 
come the  force  of  Cohesion.     Of  Adhesion.    Of  Gravity.    Apply  the 
principle  of  circular  motion  to  the  revolution  of  the  earth  about  the 
sun.     What  effect  does  the  revolution  of  the  earth  on  its  axis  have 
upon  all  bodies  on  the  surface  ? 

35.  What  would  be  the  effect  if  the  rotation  were  to  cease  ?     Describe 
the  action  of  the  centrifugal  force  on  a  hoop  rapidly  revolved  on  its 
axis.    What  is  the  Gyroscope  ?     Define  reflected  motion. 

36.  Give  its  law.     What  is  Energy,  in  the  Physical  sense  of  the  word  ? 
To  what  is  it  proportional  ?     Name  and  define  the  two  forms  of  energy. 
How  may  one  form  be  changed  into  the  other  ? 

37.  What  is   the   law  of  the  Conservation  of  Energy?    What  did 
Faraday  say  with  regard  to  this  law  (p.  40)  ? 

III.  Attraction.  I.  MOLECULAR  FORCES. — Define  a  molecular  force. 
What  two  opposing  forces  act  between  the  molecules  of  matter  ?  How 
is  this  shown?  What  is  the  repellent  force?  Name  the  attractive 
forces.  Which  of  these  belong  to  Physics  ? 

1.  COHESION. — Define.     What  are  the  three  states  of  matter?    Define. 
How  can  a  body  be  changed  from  one  state  to  another  ? 

44.  Show  that  cohesion  acts  only  at  insensible  distances.     Explain 
the  process  of  welding.     Why  cannot  all  metals  be  welded  ?    Why  do 
drops  of  dew,  etc.,  take  a  globular  form  ?    Why  do  not  all  bodies  have 
this  form  ?     Illustrate  the  tendency  of  matter  to  a  crystalline  struc- 
ture. 

45.  Has  each  substance  its  own  form  ?    Why  is  not  cast-iron  crystal- 
line ?    Why  do  cannon  become  brittle  after  long  use  ? 

46.  Describe  the  process  of  tempering  and  annealing.     Explain  the 
Rupert's  Drop.     How  is  glassware  annealed  ? 

2.  ADHESION. — Define.      What   is  the  theory  of  filtering  through 
charcoal  ?     Of  what  use  is  soap  in  making  bubbles  ?     Define  Capillary 
Attraction.     Why  will  water  rise  in  a  glass  tube,  while  mercury  will  be 
depressed  ?     Is  a  tube  necessary  to  show  capillary  attraction  ?     What 
is  the  law  of  the  rise  in  tubes  ? 

48.  Give  practical  illustrations  of  capillary  action.    Why  will  not 


QUESTIONS. 

old  cloth  shrink  as  well  as  new,  when  washed  ?  What  is  the  cause  o\ 
solution  ?  Why  is  the  process  hastened  by  pulverizing? 

49.  Tell  what  you  can  about  gases  dissolving  in  water.  Why  does 
the  gas  escape  from  soda  water  as  soon  as  drawn?  Why  do  pressure 
and  cold  favor  the  solution  of  a  gas  ?  Describe  the  diffusion  of  liquids. 
Of  gases. 

50-1.  Describe  the  osmose  of  liquids.  Of  gases.  Why  do  rose- 
balloons  lose  their  buoyancy?  What  is  the  difference  between  the 
osmose  and  the  diffusion  of  gases  ? 

II.  GRAVITATION.— How  does  Gravitation  differ  from  Cohesion  and 
Adhesion  ?  What  is  the  law  of  gravitation  ?  Why  does  a  stone  fall  to 
the  ground?  Will  a  plumb-line  near  a  mountain  hang  perpendic- 
ularly ?  Why  do  the  bubbles  in  a  cup  of  tea  gather  on  the  side  ?  How 
is  the  earth  kept  in  its  place  ?  Define  Gravitation  Gravity.  Weight. 

53.  State  the  three  laws  of  weight  What  is  a  vertical  or  plumb- 
line? 

54-5.  State  the  four  laws  of  falling  bodies.  Describe  the  "  guinea- 
and-feather  experiment."  What  does  it  prove?*  Give  the  equations 
of  falling  bodies. 

56.  How  can  the  time  of  a  falling  body  be  used  for  determining  the 
depth  of  a  well  ?  How  does  gravity  act  upon  a  body  thrown  upward  ? 
What  velocity  must  be  given  to  a  ball  to  elevate  it  to  any  point  ?  How 
high  will  it  rise  in  a  given  time  ?  When  it  falls,  with  what  force  will  it 
strike  the  ground  ?  Define  the  Centre  of  Gravity.  The  line  of  direo 
tion.  The  three  states  of  equilibrium. 

57-8.  How  may  the  centre  of  gravity  be  found  ?  Give  the  genera' 
principles  of  the  centre  of  gravity.  Describe  the  leaning  tower  of 
Pisa.  State  some  physiological  applications  of  the  centre  of  gravity 
Why  do  fat  people  always  walk  so  erect  ? 

59-60.  Define  the  Pendulum.  Arc.  Amplitude.  What  are  isochro- 
nous vibrations.  State  the  three  laws  of  the  pendulum.  Who  dis- 
covered the  first  law  ?  (See  p.  65.)  What  is  the  centre  of  oscillation?  f 
How  is  it  found  ?  What  is  the  centre  of  percussion  ? 

*  "  It  is  difficult  for  many  pupils  to  understand  how,  under  the  influence  of  gravity 
alone,  all  bodies  fall  with  equal  rapidity.  An  illustration,  which  is  usually  effective, 
is  that  of  a  number  of  bodies  of  the  same  kind,  say  bricks,  which  will  separately  fall 
in  the  same  space  of  time.  The  pupil  will  admit  that,  if  all  of  them  are  connected 
together,  inasmuch  as  nothing  is  thereby  added  to  their  weight,  there  is  no  reason 
why  the  mass  of  bricks  should  not  fall  in  the  time  of  a  single  one,  notwithstanding 
it  is  a  larger  body."—  Wm.  H.  Taylor. 

t  "  Take  a  flat  board  of  any  form  and  drive  a  piece  of  wire  through  it  near  its 
edge,  and  allow  it  to  hang  in  a  vertical  plane,  holding  the  ends  of  the  wire  by  the 
finger  and  thumb.  Take  a  small  bullet,  fasten  it  to  the  end  of  a  thread,  and  allow  the 
thread  to  pass  over  the  wire  jo  that  the  bullet  hangs  close  to  the  board.  Move  the 


QUESTIONS.  261 

61-2.  Describe  the  pendulum  of  a  clock.  How  is  a  clock  regulated  ? 
Does  it  gain  or  lose  time  in  winter?  Describe  the  gridiron  pendulum. 
The  mercurial  pendulum.  Name  the  various  uses  of  the  pendulum 
Describe  Foucault's  experiment. 

IV.  The  Elements  of  Machines.— Name  and  define  the  elements 
of  machinery.    Do  the  "  powers,"  so  called,  produce  energy?    What  is 
the  law  of  mechanics  ?  Illustrate  the  law.    What  is  a  lever  ?    Describe 
the  three  classes  of  levers.    The  law  of  equilibrium. 

71.  What  is  the  advantage  peculiar  to  each  class?  Describe  the 
steelyard  as  a  lever.  What  effect  does  it  have  to  reverse  the  steelyard  ? 
Describe  the  arm  as  a  lever.  (See  Physiology,  p.  48.)  Would  a  lever 
of  the  first  class  answer  the  purpose  of  the  arm  ?  Describe  the  com- 
pound lever. 

72-3.  The  hay  scale.  The  wheel  and  axle.  Its  law  of  equilibrium. 
Describe  a  system  of  wheel-work.  At  which  arm  of  the  lever  is  the  P. 
applied  ? 

74.  Describe  the  various  uses  of  the  inclined  plane.  Its  law  of 
equilibrium.  Wbat  velocity  does  a  body  acquire  in  rolling  down  an 
inclined  plane?  Lrive  illustrations. 

75-6.  Describe  the  screw.  Its  uses.  Its  law  of  equilibrium.  How 
may  its  power  be  increased?  What  limit  is  there?  Describe  the 
wedge.  Its  uses.  Its  law  of  equilibrium.  How  does  it  differ  from 
that  of  the  other  powers?  Describe  the  pulley.  The  use  of  fixed 
pulleys.  Is  there  any  gain  of  P.  in  a  fixed  pulley  ? 

77-8.  What  is  the  use  of  a  movable  pulley.  Describe  a  movable 
pulley  as  a  lever.  Give  the  general  law  of  equilibrium  in  a  combination 
of  pulleys.  What  part  of  the  force  is  lost  by  friction  ?  What  are 
cumulative  contrivances  ?  Is  perpetual  motion  possible  ? 

V.  Pressure  of  Liquids  and  Gases,    i.  HYDROSTATICS. — Define. 
What  liquid  is  taken  as  the  type?    What  is  the  first  law  of  liquids? 
Explain.     Illustrate  the  transmission  of  pressure  by  water.     Show  how 


hand  by  which  you  hold  the  wire  horizontally  in  the  plane  of  the  board,  and  observe 
whether  the  board  moves  forward  or  backward  with  respect  to  the  bullet.  If  it 
moves  forward,  lengthen  the  string,  if  backward,  shorten  it  till  the  bullet  and  the 
board  move  together.  Now  mark  the  point  of  the  board  opposite  the  centre  of  the 
bullet,  and  fasten  the  string  to  the  wire.  You  will  find  that,  if  you  hold  the  wire  by 
the  ends  and  move  it  in  any  manner,  however  sudden  and  irregular,  in  the  plane  of 
the  board,  the  bullet  will  never  quit  the  marked  spot  on  the  board.  Hence  this  spot 
is  called  the  centre  of  oscillation,  because,  when  the  board  is  oscillating  about  the 
wire  when  fixed,  it  oscillates  as  if  it  consisted  of  a  single  particle  placed  at  the  spot. 
It  is  also  called  the  centre  of  percussion,  because,  if  the  board  is  at  rest  and  the  wire 
is  suddenly  moved  horizontally,  the  board  will  at  first  begin  to  rotate  about  the  spot 
as  a  centre." — J.  Clerk  Maxwell,  on  Matter  and  Motion,  p.  104. 


262 

water  is  used  as  a  mechanical  power.  Describe  the  hydrostatic  press. 
Give  its  law  of  equilibrium. 

86-90.  What  are  the  uses  of  this  press?  What  pressure  is  sustained 
by  the  lower  part  of  a  vessel  of  water,  when  acted  on  by  gravity  alone  ? 
How  does  this  pressure  act  ?  State  the  four  laws  which  depend  on  this 
principle,  and  illustrate  them.  What  is  the  weight  of  a  cubic  foot  of 
sea  water?  Freshwater?  What  is  the  pressure  at  two  feet?  Give 
illustrations  of  the  pressure  at  great  depths.  Describe  the  hydrostatic 
bellows.  Its  law  of  equilibrium.  What  is  the  hydrostatic  paradox  ? 
Give  illustrations.  Give  the  principle  of  fountains.  How  high  will 
the  water  rise?  How  do  modern  engineers  carry  water  across  a  river? 
Did  the  ancients  understand  this  principle  ?  Give  the  theory  of  the 
Artesian  well,  and  of  ordinary  wells  and  springs. 

91-2.  Give  the  rule  for  finding  the  pressure  on  the  bottom  of  a  ves- 
sel. On  the  side.  Define  the  water-level.  Is  the  surface  of  water 
horizontal  ?  If  it  were,  what  part  of  an  approaching  ship  would  we 
see  first  ?  Describe  the  spirit-level.  Define  specific  gravity.  What  is 
the  standard  for  solids  and  liquids  ?  For  gases  ?  Explain  the  buoyant 
force  of  liquids. 

93.  What  is  Archimedes's  law?  (p.  119.)  Describe  the  "  cylinder- 
and-bucket  experiment."  What  does  it  prove?  Give  the  method  of 
finding  the  specific  gravity  of  a  solid. 

94-6.  A  liquid.  Is  it  necessary  to  use  a  specific  gravity  flask  hold- 
ing just  1000  ozs.  or  would  any  size  answer?  Suppose  the  solid  is 
lighter  than  water  and  will  not  sink,  what  can  you  do  ?  Explain  the 
hydrometer.  How  can  you  find  the  weight  of  a  given  bulk  of  any 
substance  ?  The  bulk  of  any  given  weight  ?  The  exact  volume  of  a 
body  ?  Illustrate  the  action  of  dense  liquids  on  floating  bodies.  Why 
will  an  iron  ship  float  on  water  ?  Where  is  the  centre  of  gravity  in  a 
floating  body?  How  do  fish  sink  at  pleasure  ? 

2.  HYDRAULICS.— Define.  To  what  is  the  velocity  of  a  jet  equal  ? 
How  is  the  velocity  found  ?  Give  the  rule  for  finding  the  quantity  oi 
water  which  can  be  discharged  from  a  jet  in  a  given  time.  What  is  the 
effect  of  tubes?  Tell  something  of  the  flow  of  water  in  rivers. 

99-102.  Name  and  describe  the  different  kinds  of  water-wheels. 
Which  is  the  most  valuable  form  ?  What  is  the  principle  of  the  Tur- 
bine? Describe  Barker's  Mill.  How  are  waves  produced  ?  Explain 
the  real  motion  of  the  water.  How  does  tse  motion  of  the  whole  wave 
differ  from  that  of  each  particle  ?  How  is  the  character  of  waves  modi- 
fied near  the  shore  ?  What  is  the  extreme  height  of  "  mountain  waves  ?" 
Define  like  phases.  Unlike  phases.  A  wave-length.  What  is  the 
effect  if  two  waves  with  like  phases  coincide?  With  unlike  phases? 
What  is  this  termed  ? 


263 

3.  PNEUMATICS. — Define.  What  principles  are  common  to  liquids 
and  gases  ?  What  gas  is  taken  as  the  type  ?  Describe  the  air-pump. 
Can  a  perfect  vacuum  be  obtained  in  this  way?  (See  p.  118.)  What  is 
the  condenser?  Its  use?  Prove  that  the  air  has  weight. 

105.  Show  its  elasticity  and  compressibility.  Describe  the  bottle- 
imps.  What  principles  do  they  illustrate?  Show  the  expansibility 
of  the  air. 

106-7.  Describe  the  experiments  with  the  hand-glass.  The  principle 
of  Hero's  fountain.  The  Magdeburg  hemispheres.  What  do  they 
prove  ?  Show  the  upward  pressure  of  the  air. 

108.  The  buoyant  force  of  the  air.  Would  a  pound  of  feathers  and 
a  pound  of  lead  balance,  if  placed  in  a  vacuum  ?  On  what  principle 
does  a  balloon  rise?  What  is  the  amount  of  the  pressure  of  the  air? 
Describe  the  experiment  illustrating  this.  Where  do  these  figures 
apply? 

109-110.  Describe  how  the  pressure  of  the  air  constantly  varies. 
Explain  Mariotte's  (called  also  Boyle's)  law.  Describe  the  barometer. 
Its  uses.  Are  the  terms  "  fair,"  "  foul,"  etc.,  often  placed  on  the  scale, 
to  be  relied  upon?  Why  is  mercury  used  for  filling  the  barometer? 
Describe  Otto  Guericke's  barometer. 

111-113.  Describe  the  action  of  the  lifting-pump.  The  force-pump. 
The  fire-engine.  Compare  the  action  of  the  lifting-pump  with  that  of 
the  air-pump.  What  is  the  siphon.  Explain  its  theory. 

114-16.  Describe  the  pneumatic  inkstand.  The  hydraulic  ram.  The 
atomizer.  Show  how  a  current  of  air  drags  with  it  the  still  atmos- 
phere. What  opposing  forces  act  on  the  air?  How  high  does  the  air 
extend  ?  How  does  its  density  vary  ? 

VI.  Acoustics. — Define.  Name  and  define  the  two  senses  of  this 
word.  May  not  the  terms  "  light,"  "  heat,"  etc.,  be  used  in  the  same 
way?  Illustrate  the  formation  of  sound  by  vibrations. 

124-5.  Show  how  the  sound  of  a  tuning-fork  is  conveyed  through 
the  air.  The  report  of  a  gun.  The  sound  of  a  bell.  The  human 
voice.  Define  a  sound-wave.  In  which  direction  do  the  molecules 
of  air  vibrate  ?  In  what  form  do  the  waves  spread  ?  Can  a  sound 
be  made  in  a  vacuum?  Can  a  sound  come  to  the  earth  from  the 
stars  ? 

126.  How  do  sounds  change  as  we  pass  above  or  below  the  sea- 
ievel  ?     Upon  what  does  the  velocity  of  sound  depend  ?    Why  is  this  ? 
At  what  rate  does  sound  travel  in  the  air?    In  water?    In  the  metals? 
In  iron  (p.  145)  ?    What  effect  does  temperature  have  on  the  velocity^ 
of  sound? 

127.  Do  all  sounds  travel  at  the  same  rate?    How  does  the  velocity 
oi  sound  enable  us  to  determine  distance?     Upon  what  does  the  in- 


264  QUESTIONS. 

tensity  of  sound  depend?     At  what  rate  does  it  diminish?    Why? 

State  wherein  the  laws  of  sound  are  similar  to  those  of  other  phe- 
nomena. What  does  this  uniformity  prove? 

128.  Explain  the  speaking-tube.  The  ear-trumpet  Describe  Biot's 
experiment  in  the  water-pipes  of  Paris.  The  speaking-trumpet.  What 
is  the  refraction  of  sound  ? 

129-30.  Define  reflection  of  sound.  What  is  the  law?  Give  some 
curious  instances  of  reflection  (p.  145).  What  is  the  shape  of  a  whis- 
pering-gallery ?  Illustrate  the  decrease  of  sound  by  repeated  reflection* 
Why  are  sounds  more  distinct  at  night  than  by  day?  Is  it  desirable  to 
have  a  door  or  a  window  behind  a  speaker?  What  causes  the  "  ring- 
ing "  of  a  sea-shell  ?  How  are  echoes  produced  ?  When  is  the  echo 
repeated  ?  Illustrate  the  decrease  of  sound  by  reflection.  What  are 
acoustic  clouds  ?  * 

*  "  The  influence  of  wind  on  the  intensity  of  sound  seems  due  to  the  fact  that, 
owing  to  obstructions  opposed  by  the  ground,  there  is  a  considerable  difference 
between  the  velocity  of  the  wind  close  to  the  ground  and  the  velocity  at  the  height 
of  a  few  feet  above  the  ground.  Thus  in  a  meadow  the  velocity  of  the  wind  at  one 
foot  above  the  surface  may  be  only  half  what  it  is  at  eight  feet  above  the  surface. 
Let  us  take  the  velocity  of  sound  at  noo  feet  per  second,  and  suppose  that  the  veloc- 
ity of  a  contrary  wind  is  10  feet  per  second  at  the  surface,  and  20  feet  per  second  at 
the  height  of  8  feet  above  the  surface.  Thus,  considering  this  circumstance  alone,  the 
wave  of  sound  at  the  end  of  a  second  would  be  at  the  surface  10  feet  in  advance  of  its 
position  at  8  feet  above  the  surface  ;  so  that  the  front  of  the  wave  instead  of  being  a 
vertical  plane  would  be  inclined  to  the  horizon.  Thus  the  sound  instead  of  proceed- 
ing horizontally  becomes  turned  upward.  It  only  remains  to  add  that  this  tilting  of 
the  front  of  the  wave  is  not  delayed  until  the  end  of  a  second,  but  begins  at  the  origin 
of  the  sound  and  increases  gradually.  Hence  a  ray  of  sound,  so  to  speak,  instead  of 
travelling  horizontally  is  curved  upwards,  and  thus  passes  over  the  head  of  a  person 
stationed  at  a  distance  from  the  origin.  A  contrary  wind  then  diminishes  the  inten- 
sity of  sound  by  lifting  the  sound  off  the  ground,  and  the  amount  of  this  lifting 
increases  as  the  distance  from  the  origin  increases.  The  various  consequences  which 
may  be  deduced  from  the  preceding  theory  have  been  verified  by  experiments.  Thus 
it  follows  that  a  listener  when  the  wind  is  contrary  may  expect  to  recover  a  sound, 
which  he  has  lost  at  a  certain  distance  from  its  origin,  by  ascending  to  some  height 
above  the  surface.  Also  the  influence  of  a  wind  will  be  but  small  if  the  surface  be 
very  smooth  •  thus  sounds  are  heard  against  the  wind  much  farther  over  calm  water 
than  over  land.  Again,  suppose  the  origin  of  the  sound  to  be  elevated  above  the 
surface:  then  if  the  listener  be  also  raised  above  the  surface  he  may  hear  a  very 
loud  sound  made  up  of  two  parts,  namely,  that  which  has  travelled  horizontally,  and 
that  which  has  been  tilted  upwards  from  the  ground  by  the  action  of  the  contrary 
wind.  Next,  suppose  the  wind  to  be  favorable  instead  of  contrary.  In  this  case  the 
higher  part  of  the  wave  of  sound  moves  more  rapidly  than  the  lower,  and  so  the 
plane  front  of  the  wave  is  tilted  forward,  and  the  rays  of  sound  are  bent  down-ward 
to  the  advantage  of  the  listener  on  the  ground.  Thus  the  influence  of  the  wind  on 
sound  has  been  shown  to  depend  on  the  circumstance  that  when  the  wind  is  blowing, 
the  velocity  of  sound  is  different  at  different  heights  above  the  ground  :  similar  effects 
will  therefore  follow  if  this  difference  of  velocity  is  produced  by  any  other  cause 
instead  of  by  the  wind.  Now  change  of  temperature  affects  the  velocity  of  sound: 
if  the  temperature  rise  one  degree  of  Fahrenheit's  thermometer  the  velocity  increases 
by  about  a  foot  per  second.  In  general,  as  we  ascend  in  the  air  during  the  day  the 


QUESTIONS. 

131-3.  What  is  the  difference  between  noise  and  music?  Upon 
what  does  pitch  depend  ?  Describe  the  siren.  How  is  it  used  to  de« 
termine  the  number  of  vibrations  in  a  sound  ?  How  is  the  octave  of 
any  note  produced  ?  How  can  we  ascertain  the  length  of  the  wave  in 
sound  ?  What  length  of  wave  produces  the  low  tones  in  music  ?  The 
high  tones  ?  Give  the  illustration  of  the  locomotive  whistle.*  When 
are  two  tones  in  unison  ?  How  can  we  find  the  length  of  the  wave  in 
any  musical  sound  ?  What  is  meant  by  the  super-position  of  sound- 
waves ? 

134.  How  can  two  sounds  produce  silence  ?  What  is  this  effect 
termed  ?  Illustrate  interference  by  means  of  a  tuning-fork.  What  are 
"  beats  "  ?  Describe  the  vibration  of  a  cord. 

135-7.  Describe  the  sonometer.  What  is  the  object  of  the  wooden 
box  ?  Give  the  three  laws  of  the  vibration  of  cords.  What  is  a  node  ? 
Describe  the  experiments  illustrating  the  formation  of  nodes.  What 
are  acoustic  figures  ?  Nodal  lines  ? 

138-140.  What  is  the  fundamental  tone  of  a  cord  ?  A  harmonic  ? 
What  causes  the  difference  in  the  sound  of  various  instruments  ?  Does 
a  bell  vibrate  in  nodes?  The  violin-case?  A  piano  sounding-board  ? 
State  the  fractions  representing  the  relative  rates  of  vibration  of  the 
different  notes  of  the  scale.  How  is  the  sound  produced  in  wind 
instruments?  How  is  the  sound-wave  started  in  an  organ-pipe  ?  In  a 
flute?  What  determines  the  pitch?  What  are  sympathetic  vibrations? 
Describe  the  resonance  globe.  What  is  a  sensitive  flame? 

141.  A  singing  flame  ?    Describe  the  phonograph.    The  ear.    What  is 

temperature  decreases,  and  therefore  so  also  does  the  velocity  of  sound.  Thus  the 
result  is  the  same  as  in  the  case  of  a  contrary  wind  ;  the  ray  of  sound  is  lifted  over 
the  head  of  a  person  on  the  ground,  so  that  the  audibility  of  the  sound  is  diminished. 
The  presence  of  vapor  in  the  atmosphere  also  affects  the  propagation  of  sound ;  the 
velocity  increases  as  the  quantity  of  vapor  increases.  The  direct  effect,  however,  is 
very  slight,  but  indirectly  the  vapor  is  of  consequence,  for  it  gives  to  the  air  a  greater 
power  of  radiating  and  absorbing  heat,  and  so  promotes  inequality  of  temperature. 
The  variation  of  temperature  is  greatest  when  the  sun  is  shining,  so  that  it  is  greater 
by  day  than  by  night,  and  greater  in  summer  than  in  winter.  Hence,  according  to 
the  theory  now  explained,  sounds  ought  to  be  heard  more  plainly  by  night  than  by 
day,  and  more  plainly  in  winter  than  in  summer.  That  sounds  are  heard  more 
plainly  by  night  than  by  day  is  a  well-known  fact.  We  have  supposed  that  the 
temperature  decreases  as  we  ascend  in  the  atmosphere  ;  but  it  may  happen  on  some 
occasion  that  the  temperature  at  the  surface  is  lower  than  it  is  a  little  above  the  sur- 
face. This  may  be  the  case  for  instance  over  the  surface  of  the  sea  in  the  day  time, 
and  over  the  surface  of  the  land  by  night.  Thus  the  effect  on  sound  will  be  similar 
to  that  of  a  favorable  wind.  It  is  obvious  that  by  the  combined  influence  of  wind 
and  temperature  the  results  produced  may  vary  much  as  to  degree  ;  for  instance,  the 
operation  of  a  contrary  wind  may  be  neutralized  by  that  of  the  temperature  rising 
as  we  ascend  above  the  surface."  See  Proceedings  of  the  Royal  Society  of  Great 
Britain,  volumes  XXII  and  XXIV. 

*  A  speed  of  40  miles  per  hour  will  sharpen  the  tone  of  the  whistle  of  an  approach- 
•jig  train  by  a  semitone. 


266  QUESTIONS. 

the  object  of  the  Eustachian  tube  ?  Is  there  any  opening  between  the 
external  and  internal  ear  ?  What  effect  does  it  have  on  the  hearing  to 
increase  or  diminish  the  pressure  of  the  air?  How  does  a  concussion 
sometimes  cause  temporary  deafness  ?  How  can  this  be  remedied  ? 
What  are  the  limits  of  hearing?  Does  the  range  vary  in  different 
persons  ?  What  sounds  are  generally  heard  most  acutely  ?  Are  there 
probably  sounds  in  nature  we  never  hear?  Has  nature  a  tendency  to 
music  ?  What  causes  the  "  whispering  of  the  pines  ?"  What  is  the  key 
of  nature  ? 

VII.  Optics. — Define.  A  luminous  body.  A  non-luminous  body. 
A  medium.  A  transparent  body.  A  translucent  body.  An  opaque 
body.  A  ray  of  light.  Show  that  neither  air  nor  water  is  perfectly 
transparent.  Why  is  the  sun's  light  fainter  at  sunset  than  at  mid-day? 
Define  the  visual  angle.  Show  how  distance  and  size  are  intimately 
related. 

150.  State  the  laws  of  light.     Do  they  resemble  those  of  sound  } 
What  is  the  velocity  of  light?     How  is  this  proved?    Explain    the 
undulatory  theory  of  light. 

151.  How  does  light-motion    differ  from   sound-motion?    What   is 
diftased  light  ?    Why  are  som  3  objects  brilliant  and  others  dull  ?    Why 
can  we  see  a  rough  surface  at  any  angle,  and  an  image  in  the  mirror  at 
only  a  particular  one  ?     Would  a  perfectly  smooth  mirror  be  visible  ? 
How  does  reflection  vary?     Define  mirrors.     Name  and  define  the 
three  kinds. 

152.  What  is  the  general  principle  of  mirrors  ?    Why  is  an  image  in 
a  plane  mirror  symmetrical  ?     Why  is  it  reversed  right  and  left  ?    Why 
is  it  as  far  behind  the  mirror  as  the  object  is  before  it? 

153-6.  Why  can  we  often  see  in  a  mirror  several  images  of  an  object? 
Why  can  we  see  these  best  if  we  look  into  the  mirror  very  obliquely  ? 
Why  is  an  image  seen  in  water  inverted  ?  When  the  moon  is  near  the 
meridian,  why  can  we  see  the  image  in  the  water  at  only  one  spot  ? 
When  do  we  see  a  tremulous  line  of  light  ?  WThat  is  the  action  of  a 
concave  mirror  on  rays  of  light?  Define  the  focus.  Centre  of  curva- 
ture. Focal  distance.  Describe  the  image  seen  in  a  concave  mirror. 
What  are  conjugate  foci?  Describe  the  image  seen  in  a  convex  mir- 
ror. Why  is  it  smaller  than  life  ?  Why  can  it  not  be  inverted  like  one 
seen  in  a  concave  mirror  ?  Define  total  reflection. 

157.  Define  Refraction.     Does  the  partial  reflection  of  light  as  it 
passes  from  one  medium  to  another  of  different  density  have  a  parallel 
in  sound  ?    Why  is  powdered  ice  opaque  while  a  block  of  ice  is  trans- 
parent?   Give  illustrations  of  refraction. 

158.  Why  does  an  object  in  water  appear  to  be  above  its  true  place  ? 
What  is  the  general  principle  of  refraction  ?    State  the  laws  of  refrac- 


QUESTIONS.  267 

tion.     Describe  the  path  of  a  ray  through  a  window-glass.     Is  the 

direction  of  objects  changed  ?     Describe  the  path  through  a  prism. 

159-61.  Name  and  describe  the  different  kinds  of  lenses.  What  is 
the  effect  of  a  double-convex  lens  on  rays  of  light  ?  What  is  this  kind 
of  lens  often  called  ?  Describe  the  image.  Why  is  it  inverted  after  we 
pass  the  principal  focus  ?  Why  is  it  decreased  in  size  ?  What  is  the 
effect  of  a  double-concave  lens  on  rays  of  light  ?  Describe  the  image. 
Why  can  it  not  be  inverted  like  one  through  a  double-convex  lens  ? 
Describe  the  images  seen  in  the  large  vases  in  the  windows  of  drug- 
stores. What  is  Aberration  ?  * 

162.  What  is  a  mirage?     Give  its  cause. 

163.  How  is  the  solar  spectrum  formed  ?     Name  the  seven  primary 
colors.      Show  that  these  seven  will  form  white  light.     Why  are  the 
rays  separated  ?     What  is  meant  by  the  dispersive  power  of  a  prism  ? 
What  apparatus  possesses  this  property  in  a  high  degree  ?    Ans.  A 
triangular  bottle  filled  with  a  liquid  called  carbon  disulphide  (Chem- 
istry, p.  118).     What  three  classes  of  rays  compose  the  spectrum?     Do 
artificial  lights  differ  in  their  proportion  of  these  rays  ?     Why  does  the 
window  of  a  photographer's  dark   room   sometimes   contain   yellow 
glass  ? 

164-7.  Describe  the  three  kinds  of  spectra.  The  spectroscope. 
What  are  its  uses  ?  Describe  rainbows — primary  and  secondary.  Why 
is  the  rainbow  circular  ?  How  is  the  rainbow  formed  ?  Why  must  it 
rain  and  the  sun  shine  at  the  same  time,  to  produce  the  bow  ?  vVhy 
is  the  bow  in  the  sky  opposite  the  sun  ?  How  many  refractions  and 
reflections  form  the  primary  bow  ?  The  secondary  ?  How  many  colors 
can  one  receive  from  a  single  drop?  Define  complementary  colors. 
How  can  they  be  seen?  What  is  the  effect  of  complementary  colors 

*  To  prevent  spherical  aberration  the  pupil  o*.  the  eye  can  be  made  very  small. 
The  photographer  reaches  the  same  result  by  Ihe  use  of  a  diaphragm  with  a  small 
aperture.  "  The  power  of  a  small  orifice  to  correct  the  greatest  amount  of  distortion 
from  interfering  rays  is  shown  by  a  simple  experiment.  The  normal  eye  of  an  adult 
cannot  see  to  read  small  print  nearer  than  six  inches.  Within  that  distance  the  type 
becomes  more  indistinct  the  closer  it  approaches  the  eye.  But  if  we  make  a  pinhole 
through  a  card  and  place  it  close  to  the  eye,  we  can  see  to  read  printed  matter  of  any 
size  even  as  near  as  half  an  inch  from  the  eye.  At  that  distance  we  can  see  even  the 
texture  of  fine  cambric  with  microscopic  definition.  The  cause  of  this  is  easily  expli- 
cable. The  rays  striking  the  lens  perpendicularly  on  the  centre  suffer  no  refraction. 
The  effect  of  the  pinhole  is  to  exclude  all  rays  but  tho^e  that  impinge  perpendicularly 
on  the  centre  of  the  eye  lenses.  Hence  the  image  of  the  object  close  in  front  of  the 
eye  is  pictured  on  the  retina  without  the  interference  of  the  surrounding  rays,  which 
would  fall  obliquely  on  the  lens,  and  being  retracted  out  of  focus  would  blur  thft 
picture.  Observation  of  the  effect  of  a  small  orifice  in  correcting  aberrant  rays, 
and  of  the  fact  that  the  pupil  contracts  in  near  vision,  led  Haller  and  some  other 
physiologists  to  believe  that  contraction  of  the  pupil  was  the  sole  factor  in  near 
accommodation.  But  this  view  has  been  sufficiently  refuted  by  other  observers."^- 
Dr.  Dudgeon's  "  Human  Eye"  p.  76- 


266  QUESTIONS. 

when  brougiit  in  contrast  ?  (In  Fig.  163  opposite  colors  are  compie- 
mentary.)  Why  do  colors  seen  by  artificial  light  appear  differently 
than  by  day-light— as  yellow  seems  white,  blue  turns  to  green,  etc. 

168.  Describe  Newton's  rings.     How  are  these  explained  according 
to  the  wave  theory  ?     What  causes  the  play  of  color  in  mother-of-pearl? 
In  soap-bubbles  ?     In  the  scum  on  stagnant  water  ?     In  thin  layers  of 
mica  or  quartz  ? 

169.  What  can  you  say  about  the  length  of  the  waves  ?     State  the 
analogy  between   color  and    pitch  in  music.     Why  is  grass  green? 
When  is  a  body  white?     Black  ?     What  is  color-blindness  ? 

170.  What  is  double  refraction?     What  are  the  two  rays  termed? 
What  is  polarized  light?     How  does  a  dot  appear  through  Iceland 
spar  ?    What  other  methods  are  there  of  polarizing  light  ?     State  some 
illustrations  and  practical  uses  of  polarized  light. 

171.  What  is  the  meaning  of  the  word  microscope?     Describe  the 
simple  microscope.     The  compound  microscope.     How  is  the  power 
of  a  microscope  indicated  ?     Do  we  see  the  object  directly  in  a  micro- 
scope ?    Why  is  the  object-lens  made  so  small  and  so  convex? 

172-3.  What  is  the  meaning  of  the  word  telescope?  Describe  the 
reflecting  telescope.  The  refracting  telescope,  What  is  the  use  of  the 
object  lens?  The  eye-piece?  Is  the  image  inverted?  Describe  the 
opera-glass. 

174.  The  stereoscope.  The  magic  lantern.  How  are  dissolving 
views  produced  ? 

175-7.  Describe  the  Camera.  The  structure  of  the  eye.*  The 
formation  of  an  image  on  the  retina.  The  adjustment  of  the  eye.  The 
cause  of  near  and  far  sightedness.  The  remedy.  Why  do  old  people 
hold  a  book  at  arm's  length  ?  Illustrate  the  duration  of  an  impression. 
What  is  the  range  of  the  eye  ? 

*  "  In  the  skate's  eye,  and  generally  in  the  eyes  of  fishes,  the  cornea  is  nearly  quite 
dat,  the  aqueous  humor  is  insignificant,  and  there  is  virtually  no  anterior  chamber, 
for  the  crystalline  lens  comes  up  close  to  the  cornea.  A  convex  cornea  filled  by  an 
aqueous  humor  would  be  of  no  use  in  the  wat_r,  the  refractive  index  of  the  water 
being  identical  with  that  of  the  aqueous  humor.  Accordingly  the  refraction  of  the 
rays  of  light  has  to  be  effected  entirely  by  the  crystalline  lens,  which  is  nearly 
spherical,  and  of  much  greater  refractive  power  than  the  corresponding  organ  in 
animals  which  pass  their  lives  in  the  air.  The  crystalline  lens  being  so  nearly  spher- 
ical in  shape  and  of  such  high  refractive  power,  the  axis  of  the  eye  is  short.  The  eye 
of  the  turtle  which  is  so  much  in  the  water,  is  very  similar  to  that  of  the  fish.  The 
crystalline  lens  is  very  near  the  cornea.  The  lens  is  smaller  proportionally  than  that 
of  the  skate,  nor  is  it  nearly  so  spherical ;  and  its  density,  and  consequently  its  refrac- 
tive power,  is  somewhat  less.  Hence  it  has  proportionally  a  longer  focus.  The 
cornea  is  more  convex  than  that  of  the  skate.  The  fish  having  no  eyelids  nor  any 
lachrymal  apparatus,  its  cornea  \  ill  be  apt  to  become  dim  by  exposure  to  the  air,  but 
the  turtle  is  well  supplied  with  the  requisite  apparatus  for  maintaining  the  trans- 
parency of  the  eye  in  air.  Ophidiar  reptiles  have  no  eyelids  or  lachrymal  apparatus, 
but  they  do  not  require  them,  as  xheir  cornea  is  transparent  though  dry."— Dr. 
Dudgeon'*  "  Human  Eye"  p.  «o 


VIII.  Heat.— Define  luminous  heat.  Obscure  heat.  A  diather- 
manous  body.  (See  p.  205.)  Cold.  Gases  and  vapors.  Show  the 
intimate  relation  between  light  and  heat.  What  is  light?  How  do 
the  three  classes  of  rays  in  the  solar  spectrum  differ  ?  What  effect 
does  each  of  these  produce  ?  What  is  the  theory  of  heat  ?  Why  can 
we  not  see  with  our  fingers  or  taste  with  our  ears  ?  At  what  rate  does 
aerve-motion  travel?  (See  Physiology,  p.  159.)  How  long  does  it 
lake  a  tall  man  to  find  out  what  is  going  on  in  his  foot  ? 

185-7.  Name  the  sources  of  heat.  Describe  and  illustrate  each  of 
these.  Can  force  be  destroyed  ?  If  apparently  lost,  what  becomes  of 
it?  What  is  Joule's  law  ?  Define  latent,  sensible,  and  specific  heat. 
Explain  the  paradox,  "  that  freezing  is  a  warming  process  and  thawing 
a  cooling  one."  Why  does  "heat  expand  and  cold  contract"?  What 
do  you  say  as  to  the  expansion  of  solids,  liquids,  and  gases  ?  Illustrate 
the  expansion  of  solids.  Is  it  better  to  buy  alcohol  in  summer  or  ir» 
winter?  What  is  the  thermometer?  Describe  it.  Describe  the  pro. 
.  cess  of  filling  and  grading.  The  F.,  C.,  and  R.  scales.  Tell  what  you  can 
about  liquefaction.  Of  a  solid.  Of  a  gas.  In  one  case  sensible  heat 
becomes  latent,  in  the  other  latent  heat  becomes  sensible — why  is  this  ? 

188-90.  Explain  how  a  freezing  mixture  "  makes  ice  cream."  State 
the  theory  of  vaporization.  Of  distillation.  Since  rain  comes  from  the 
ocean,  why  is  it  not  salt  ?  Describe  the  theory  of  boiling.  What  is  the 
boiling  point  ?  Do  a1!  liquids  boil  at  the  same  temperature  ?  What 
would  be  the  effect,  if  this  were  the  case  ?  Upon  what  does  the  boiling- 
point  depend  ?  Why  does  pressure  raise  the  melt.  pt.  of  most  sub- 
stances but  lessen  that  of  ice  (See  notes,  pp.  190  and  202)?  Why  does 
salt-water  boil  at  a  higher  temperature  than  fresh-water  ?  Why  will 
milk  boil  over  so  easily  ?  Why  will  soup  keep  hot  longer  than  boiling 
water?  Does  the  air  dissolved  in  water,  have  any  influence  on  the 
boiling-point?  (p.  202.)  Can  you  measure  the  height  of  a  mountain  by 
means  of  a  tea-kettle  and  a  thermometer?  Show  how  cold  water  may 
be  used  to  make  warm  water  boil?  At  what  temperature  will  water 
boil  in  a  vacuum?  Why?  Can  we  heat  water  in  the  open  air  above 
the  boiling-point  ?  What  becomes  of  the  extra  heat  ?  What  is  the 
latent  heat  of  water  ?  Upon  what  principle  are  buildings  heated  by 
steam  ?  Have  you  ever  seen  any  steam  ? 

191.  Define  evaporation.  Does  snow  evaporate  in  the  winter?  What 
can  be  done  to  hasten  evaporation  ?  Why  is  a  saucepan  made  broad  ? 
Why  do  we  cool  ourselves  by  fanning?  Why  does  an  application  of 
spirits  to  the  forehead  allay  fever  ?  Why  does  wind  hasten  the  drying 
of  clothes?  Describe  a  vacuum-pan.  Why  is  evaporation  hastened  in 
a  vacuum  ?  Why  is  evaporation  a  cooling  process  ?  How  is  ice  manu- 
tactured  in  the  tropics  ?  What  is  the  spheroidal  state  ? 

192-3.  Name  and  define  the  three  modes  of  communicating  hea* 


270  QUESTIONS. 

Give  illustrations  showing  the  relative  conducting  power  of  solids, 
liquids,  and  gases.  What  substances  are  the  best  conductors  ?  Is 
water  a  good  conductor  ?  Air?  What  is  the  principle  of  ice-houses? 
Fire-proof  safes  ?  Why  do  not  flannel  and  marble  appear  to  be  of  the 
same  temperature  ?  Is  ice  always  of  the  same  temperature  ?  Describe 
the  convective  currents  in  heating  water.  Where  must  the  heat  be 
applied  ?  Where  should  ice  be  applied  in  order  to  cool  water  ?  De- 
scribe the  convective  currents  in  heating  air.  Upon  what  principle 
are  hot-air  furnaces  constructed  ?  Ought  the  ventilator  at  the  top  of  a 
room  to  be  opened  in  winter  ?  At  the  bottom  ?  Is  space  warmed  by 
the  sunbeam  ? 

194.  Does  the  heat  of  the  sun  come  in  through  our  windows  ?    Does 
the  heat  of  our  stoves  pass  out  in  the  same  way  ?    Show  how  the  vapor 
in  the  air  helps  to  keep  the  earth  warm.    Explain  the  Radiometer.   The 
relation  between  absorption  and  reflection. 

195.  What  is  the  elastic  force  of  steam  at  the  ordinary  pressure  of  the 
air  ?    What  is  the  difference  between  a  high-pressure  and  a  low-pres- 
sure engine  ?     Which  is  used  for  a  locomotive  ?    Why  ?     Describe  the 
governor.    What  is  the  object  of  a  fly-wheel  ? 

197.  How  does  the  capacity  of  the  air  tor  moisture  vary  ?  What  is 
the  principle  on  which  dew,  rain,  etc.,  depend?  Show  that  a  change  in 
density  produces  a  ch?Jige  in  temperature.  Whit  effect  does  this  have 
on  the  temperature  of  elevated  regions?  Does  an  ounce  of  air  on  a 
mountain-top  contain  the  same  quantity  of  heat  as  the  same  weight  at 
the  foot  ?  How  is  dew  formed  ? 

198-9.  Upon  what  oojects  will  it  collect  most  readily?  Why  will 
it  not  form  on  windy  nights  ?  Why  is  rice-straw  used  in  Bengal  in 
making  ice  ?  V/hat  is  a  fog  ?  Why  do  fogs  form  over  ponds  in  the 
early  evening?  Cause  of  fogs  over  the  Newfoundland  banks?  How 
does  a  fog  differ  from  a  cloud  ?  Why  do  clouds  remain  suspended  in 
the  air,  contrary  to  gravity  ?  Describe  the  different  kinds  of  clouds. 
Describe  the  formation  of  rain.  Snow. 

200-3.  How  are  winds  produced  ?  Land-and-sea  breezes  ?  Trade- 
winds?  Oceanic  currents?  Tell  about  the  Gulf  Stream.  Explain  the 
influence  which  water  has  on  climate.  Of  what  practical  use  is  the 
air  in  water?  Describe  the  exception  wh:ch  exists  in  the  freezing 
of  water.  Why  is  this  made  ?  Describe  the  two  processes  by  which 
pure  water  can  be  obtained.  How  is  an  excessive  deposit  of  dew 
prevented  ? 

IX.  Electricity. — Give  the  origin  of  this  word.  Name  the  different 
kinds  of  Electricity.  Define  Magnetism.  A  Magnet.  A  natural 
magnet  An  artificial  one.  A  bar-magnet.  A  horse-shoe  magnet. 
The  poles.  The  magnetic  curves.  Describe  a  magnetic  needle.  What 


QUESTIONS.  271 

is  the  law  of  magnetic  attraction  and  repulsion  ?  Define  magnetic  in- 
duction.  Explain  it. 

213.  When  is  a  body  polarized  ?  Give  some  illustrations  of  induced 
magnetism.  Does  a  magnet  lose  any  force  by  induction  ?  How  do 
you  explain  the  fact  that  if  you  break  a  magnet  each  part  will  have  its 
N.  and  S.  poles. 

214-15.  Describe  the  process  of  making  a  magnet.  On  what  principle 
will  you  explain  this  ?  Describe  the  compass.  Is  the  needle  true  to 
the  pole?  What  causes  it  to  vary?  What  is  the  line  of  no  variation  ? 
Declination  ?  Why  does  the  needle  point  N.  and  S.  ?  What  is  a  dip- 
ping-needle ?  Explain.  How  is  a  needle  balanced  ? 

216-17.  Where  is  the  N.  magnetic  pole  ?  How  would  one  know  when 
he  reached  it  ?  Does  the  earth  induce  magnetism  ?  Which  end  of  an 
upright  bar  will  be  the  S.  pole?  How  has  the  loadstone  become 
polarized  ?  Define  frictional  electricity.  The  electroscope.  Differ- 
ence between  static  and  dynamic  electricity.  Show  the  existence  of 
two  kinds  of  electricity.  Give  the  names  applied  to  each. 

319.  State  the  law.  What  is  the  theory  of  electricity?  Is  it  a  polar 
force  ?  Is  it  easily  disturbed  ?  Define  a  conductor.  An  insulator. 

220-3.  What  is  the  best  conductor?  Best  insulator?  Is  a  poor 
conductor  a  good  insulator  ?  When  is  a  body  said  to  be  insulated  ? 
Can  electricity  be  collected  from  an  iron  rod  ?  Describe  a  plate-glass 
electrical-machine.  What  is  the  use  of  the  chain  at  the  negative  pole? 
Dtiscribe  Holtz's  electrical  machine.  Define  electrical  induction. 
State  Faraday's  theory. 

224-5.  What  is  the  relation  between  induction  and  attraction  and 
repulsion?  Describe  the  electric  chime.  Explain.  Describe  the 
dancing  images.  The  Leyden  jar.  What  gives  the  color  to  the  spark  ? 
How  is  the  jar  discharged  ? 

226-7.  What  are  the  essentials  of  a  Leyden  jar  ?  What  is  the  object 
of  the  glass  ?  The  tinfoil  ?  State  the  theory  of  the  charging  of  the 
jar.  Can  an  insulated  jar  be  charged  ?  Is  the  electricity  on  the  sur- 
face or  im  the  glass  ?  Can  the  inner  molecules  of  a  solid  conductor  be 
charged  ?  Will  a  rod  contain  any  more  electricity  than  a  tube  ?  Why 
is  the  prime  conductor  of  an  electrical-machine  hollow?  What  is  the 
effect  of  points?  Describe  the  electric  whirl.  Explain  the  existence 
of  electricity  in  the  atmosphere.  What  is  the  cause  of  lightning? 
Thunder?  Is  there  any  danger  when  you  once  hear  the  report? 
Describe  the  different  kinds  of  lightning.  Tell  how  Franklin  discov- 
ered the  identity  of  lightning  and  frictional  electricity.  (See  p.  251.) 

228-9.  Wha*  is  the  cause  of  the  Aurora  Borealis?  How  is  this 
shown  ?  Prove  the  intimate  relation  between  the  aurora  and  mag- 
netism. Tell  what  you  can  about  lightning  rods.  In  what  consists 
the  main  value  of  the  rod?  Does  the  lightning  ever  pass  upward 


QUESTIONS, 

from  the  earth?  Ans.  It  does,  both  quietly  and  by  sudden  discharge 
Has  Nature  provided  any  lightning-rods  ?  What  is  St.  Elmo's  fire  ? 
What  is  the  velocity  of  electricity  ?  Illustrate  its  instantaneous- 
ness. 

230-1.  Name  some  of  the  effects  of  frictional  electricity — (i)  Phys- 
ical, (2)  Chemical,  (3)  Physiological.  How  are  galvanic  electricity  and 
chemistry  related  ?  Why  is  galvanic  or  voltaic  electricity  thus  named  r 
Tell  the  story  of  Galvani's  discovery.  (See  p.  251.)  What  was  his 
theory?  Give  an  account  of  Volta's  discovery.  How  can  we  form  a 
simple  pile  ?  Describe  the  simple  galvanic  circuit. 

232.  Define  the  poles.     Electrodes.     Closing  and  breaking  the  cir- 
cuit.    What   is  necessary  to   form   a   voltaic   pair  ?      Are   the   terms 
applied  to  the  metals  the  same  as  those  to  the  poles?     Describe  the 
chemical  change.     Why  does  the  hydrogen  come  off  from  the  copper? 
Tell  what  you  can  about  the  current. 

233.  What  really  passes  along  the  wire  ?     How  is  this  force  trans- 
mitted?   Will   a  tube,  then,  convey  as  much  electricity  as  a  rod? 
Explain  the  term  electric  potential. 

234-5.  Describe  Smee's  battery.  Grove's  battery.  The  chemical 
change  in  this  battery.  What  are  the  advantages  of  Grove's  battery? 
Describe  Bunsen's  battery.  Daniell's  battery.  The  sulphate  of  copper 
battery.  Define  quantity  and  intensity.  Upon  what  do  they  depend  ? 
Compare  frictional  and  galvanic  electricity. 

236-9.  State  the  effects  of  galvanic  electricity,  (i)  Physical — heat 
and  light ;  (2)  Chemical— decomposition  of  water,  electrolysis,  electro- 
typing,  electro-plating,  etc. ;  (3)  Physiological. 

240.  What  is  the  effect  of  a  voltaic  current  on  a  magnetic  needle  ? 
What  is  a  galvanometer  ?  An  astatic  needle  ?  An  electro-magnet  ? 
A  helix  ?  Show  how  a  helix  can  be  magnetized.  How  are  bar-magnets 
made?  How  is  motion  produced  by  electricity?  Describe  Page's 
rotating-machine.  What  is  the  principle  of  an  electric  engine  ?  What 
difficulty  is  there  in  its  practical  use  ?  Describe  the  magnetic  tele- 
graph. How  is  a  message  sent  ?  How  is  one  received  ?  What  is  a 
sounder?  What  is  the  general  principle  of  the  telegraph?  Describe 
the  relay.  Name  the  use  of  each  instrument.  Define  magnetic  elec- 
tricity. Describe  a  magneto-electric  machine.  Describe  Wild's 
machine.  What  are  induced  currents  ?  Describe  the  Telephone. 
The  Microphone.  What  is  the  difference  between  the  acoustic  and  the 
magnetic  telephone  ?  Explain  Ruhmkorff 's  coil.  Thermal  electricity. 
A  thermo-electric  pile.  Describe  the  electric  fish. 


TABLES. 


Prepared  by  Dr.  WM.  H.  TAYLOR,  State  Assayer  and  Chemist,  and  Professor  In 
High  School,  Richmond,  Va. 


I.    LAWS    OF    FALLING    BODIES. 

In  terms 
of  16  feet. 

Velocity  at  end  of  1st  second  =  32  =  32  =  2  x  16. 

"     2d      "       =32  +  32  =  64  =  4  x  16. 

"     3d      "       =  32  +  32  +  32  =  96  =  6  x  16. 

The  constant  increase  given  by  gravity  for  every  second  is  32  feet. 

Distance  for  any  second  equals  the  mean  Letween  velocity  at  the 
beginning  and  velocity  at  the  end  of  that  second. 

_  v  at  beginning  +  v  at  end 

Hence, 

Distance  for  1st  second  =  — J —  =  16  =  1  x  16. 


..  8d     «      =     -        =  80  =  5  x  16. 


274 


TABLES. 


II.    ANALYSIS  OF  THE  MOTION  OF  A  FALLING  BODY. 


1  space. 
1st  sec.  |    Acquired  velocity  =  2  spaces. 

3  spaces. 


3d  sec 


sec. 


4th  sec. 


Acquired  velocity  =  4  spaces. 


Acquired  velocity  =  6  spaces. 


7  spaces. 
Acquired  velocity  =  8  spaces. 


1  space  =  16  ft. 

Acquired  velocity  is  such  as  would 
carry  the  body  over  twice  the 
space  already  passed  in  same 
length  of  time,  without  farther 
aid  from  gravity,  Since  grav- 
ity does  aid  it,  however,  to  the 
extent  of  16  ft.  a  second,  we 
must  include  its  aid  in  calcu- 
lating the  entire  space  passed 
over.  Ex. :  At  the  end  of  3 
seconds  the  body  has  passed 
over  1  +  3  +  5  =  9  spaces. 
Twice  this  =  18  spaces  for  the 
3  seconds,  or  6  spaces  for  one 
second.  These  6  spaces  will 
be  utilized  during  the  next 
(4th)  second,  and,  in  addition, 
gravity  will  furnish  one  space, 
making  7  spaces  through 
which  the  body  will  move 
during  the  4th  second. 


III.     SECOND  LAW  OF  PENDULUMS. 

Time  of  vibration  =  1  second.         =  2  seconds.  =  3  seconds. 

Length  =  39.1  in. 

Length  =4x89.1  in. 


Length  =  9  x  39.1  in. 


BLACKBOARD     DRAWINGS. 

(Copyright,  1878.) 


F*g*  3« — Wire-drawing  Machine.         \       Pig.    7.  -Reaction  Balls. 
Fi&  &— Torsion  Balance.  Pig.    8.— Compound  Motion. 

Fif.  &— Simultaneous  Forces.  -"tf.  15.— EartVs  Motion. 


BLACKBOARD    DRAWINGS. 


Fig.  9. — Parallelogram  of  Forces. 
Fisrs.  10  to  13. — Resolution  of  Forces. 


FORCE  :      ATTRACTION. 


277 


Fig.  16. — Centrifugal  Force. 
Fig.  17,—  The  Gyroscope. 


Fig.  18.— Reflected  Motion. 

Figs.  21  to  23. — Capillary  Attraction, 


278 


BLACKBOARD    DRAWINGS. 


Fig.  24. — Diffusion  of  Liquids. 
Fig.  25.— Diffusion  of  Gases. 
Fig.  26.— Osmose  of  Liquids. 


Fig.  27. — Osmose  of  Gases. 

Fig.  32.— Stable  Equilibrium. 

Fig.  33-—  To/ind  Center  of  Gravity, 


ATTRACTIONS  I     LEVERS. 


279 


&'*£•  ^-—Expansibility  of  Air. 
Fig.  96.— Hero's  Fountain. 


Fig.  100. — Upward  Pressure  of  Air. 
Figs.  106,  107,  108. — Lifting  Pump. 


280 


BLACKBOARD    DRAWINGS. 


Figs.  44  and  ^.  —  The  Steely  ani. 
Fig,  46. — Compound  Lever. 
F*g-  47-~" Hay-scales. 


Fig.  48.—  Wheel  and  Axle, 
Fig.  52 . —  Wh  eehvork, 


ELEMENTS     OF    MACHINES. 


281 


Fig.  54. — Inclined  Plane, 
Fig.  57.— Fixed  Pulley. 


Figs.  59  and  60.— Movable  Pulleys. 
Figs.  61  tot-^.— Combinations  of  Pulleys. 


282 


BLACKBOARD    DRAWINGS. 


Figs.  65,  66,  67,  70. —  Transmission  of  Pressure  by  Liquids. 


HYDROSTATICS. 


283 


lillll 


Fig.  Ti.-^Hyrauc     ress. 

Figs.  72  find  73.— Equal  Pressure  of  Liquids. 


284 


BLACKBOARD    DRAWINGS. 


Figs.  74  and  75. — Hydrostatic  Bellows. 
Fig-  77- — Theory  of  a  Fountain. 
Fig.  ft.— Theory  of  a  Well. 


HYDRODYNAMICS  :     PNEUMATICS. 


285 


Fig.  81. — Buoyant  Force  of  Liquids. 
Fig.  Iz.-Cylinder-and-Bucket  Experi- 
ment. 


Fig.  88.—  Turbine  Wheel. 

Fig.  89.— Barkers  Mill  and  Whirl-i-gig. 

Fig.  91. — Air  Pump. 


286 


BLACKBOARD    DRAWINGS. 


Fig.  36.— Clock  Pendulum. 
Fig.  37. — Gridiron  Pendulum. 


Fig.  38.— Foucaulfs  Experiment. 
Figs.  39  to  41.—  Three  Classes  of  Levers 


PNEUMATICS. 


28? 


Fig.  109. — Force  Pump. 
Fig.  no.— Fire  Engine. 


in. — Siphon. 

112. — Pneumatic  Inkstand, 


288 


BLACKBOARD    DRAWINGS. 


Fig.  113. — Hydraulic  Ram. 
Fig.  115. — Adhesion  of  Air  to  a   Current 
of  Air. 


Fig.  117. — Sound  Waves. 

Fig.  120. — Refraction  of  Sound- 


ACOUSTICS. 


289 


Fig.  121. — Siren. 

Fig.  125. — Vibrations  of  a  Cord. 

Fir.  126.  —  Sonometer 


Fig.  131.— Modes  of  a  Bell. 
Fig.  132.— Organ  Pipe. 


290 


BLACKBOARD    DRAWINGS. 


I3S-- -Visual Angle. 

i^.—  Tnfnd  Velocity  of  Light. 


>  I37-— Plane  Mirror. 

s.  nS.  i-v}.-."-?fi*le  Images. 


OPTICS. 


291 


Fig.  140. — Images  in  Water. 

Fig.  ni.—Concave  Mfrror. 

Fig.  142. — Image  in  Concave  Mirror. 


BLACKBOARD    DRAWINGS. 


iiiiiiiiiiiiiiiiiiiinu 


Fig.  143.  —  Conjugate  Foci. 
Fig.  144.  —Convex  Mirror. 
Fig.  145. —  Total  Reflection. 
Figs,  147,  148. — Refraction  of  Light  by 
Water. 


Fig.  149.— Refraction  of  Light  by 
Window  Glass. 

Fig.  150.-  Refraction  of  Light  by  a 
Prism, 


OPTICS. 


293 


Fig.  151. — Classes  of  Lenses. 

Fig.  152  —Double  Convex  Lens. 

Fig-  153-— Object  Magnified  by  Con-vex  Lens. 

Fig.  154.— Object  Diminished  by  Convex  Lens. 


BLACKBOARD    DRAWINGS. 


F*g-  iSS-— Double  Concave  Lens. 

Fig.  156. — Object  Diminished  by  Concave  Lens. 

Fig.  157. — Mirage. 

Fig.  159. — Solar  Spectrum. 


OPTICS. 


295 


Fig.  161. — Spectroscope. 

Fig.  162. — Rainbow. 

Fig.  163. — Complementary  Colors. 


290 


BLACKBOARD     DRAWINGS. 


Fig.  168. — Microscope. 
Fig.  169.—  Telescope. 


Fig.  ITS..— Opera  Glass. 
Fig.  172. — Stereoscope. 


OPTICS  :     HEAT. 


297 


Fig.  174. — Magic  Lantern. 
Fig,  175. — Camera  Okscttra. 


Fig.  176. — Eye. 

Fig.  177. —  Thermometers* 


298 


BLACKBOARD    DRAWINGS. 


Fig.  183. — Steam  Engine  Cylinder  and  Piston. 

Fig.  184. — Governor. 

Figs.  195  and  196.—  Polarity  of  the  Needle. 


HEAT  :     ELECTRICITY. 


299 


Fig.  201.— Plate  Glass  Electrical  Machine. 

Fi^.  zoz.—Holtz's  Electrical  Machine. 

Fig.  205.— Passage  of  Electricity  by  Polarization. 


300 


BLACKBOARD    DRAWINGS. 


Fig,  206.— Electric  Chimes. 

Fig-  2og. — Section  of  Side  of  Ley  den  Jar, 


Fig,  213.— Simple  Galvanic  Circuit. 
Fig.  214.  — Source  of  the  Hydrogen, 


INDEX. 


The  index  figures  denote  the  page. 


Aberration,  154,  161. 
Achromatic,  162. 
Acoustics,  123. 

"  clouds,  130. 

u  figures,  137. 

Action  and  reaction,  30. 
Adhesion,  47. 
Air,  103. 

Air-pump,  103,  118. 
Alcoholmeter,  94. 
Amplitude,  59. 
Animalcules,  13. 
Annealing,  46. 
Archimedes,  80,  108,  119,  180. 

Law  of,  93,  117, 119. 
Aristotle,  22,  80,  119,  206. 
Artesian  weiis,  £9. 
A  tmosphere, 
Atomic  theory,  13. 
Atomizer,  114. 
Attraction,  42. 

of  adhesion,  47. 
cohesion,  43. 
Capillary,  47. 
Gravitation,  52. 
Aurora  borealis,  228. 
Avogadro's  law,  23. 

B 

Bacon,  23,  206. 
Barker's  mill,  100. 
Barometer,  no. 
Battery,  Bunsen's,  234, 

Grove's,  234. 

Sulphate  of  copper,  235. 
41         Thermo-electric,  249. 
Beats,  134, 145. 
Bell,  125. 
Boiling,  189. 
Boyle's  law,  118. 
Brittleness,  21. 
Britannia  bridge,  86. 


Caloric,  207. 
Camera,  175. 
Capillarity,  47. 
Capstan,  73. 

Cartesian  diver,  105,  118. 
Caustic,  154. 
Centre  of  gravity,  52. 
"  oscillation,  60. 

"  percussion,  61. 

Centrifugal  force,  33. 
Chemical  affinity,  43. 
"         change,  14. 
Chromatic  aberration,  rei. 
Clepsydra,  66. 
Clock,  61. 
Clouds,  198. 
Cohesion,  43. 
Coils,  Induction,  247. 
Color,  169. 

u       blindness,  169. 

"        Prismatic,  163. 

"       Complementary,  167. 
Compass,  214. 

Compensation  pendulum,  61. 
Condenser,  164. 
Conductors,  219. 
Cords,  134. 

Correlation  of  forces,  37,  40,  207, 
Crystals,  45. 

Cumulative  contrivances,  78. 
Current,  Voltaic,  232. 
"        of  rivers,  98. 
Curves,  Magnetic,  an. 


Declination,  215. 
Democritus,  23,  206. 
Dew,  197. 
Dialysis,  51. 
Diathermancy,  205. 
Dichroic,  167. 


302 


INDEX. 


Diffraction,  168. 
Diffusion  of  liquids,  49. 
gases,  49. 

Dissolving  views,  174. 
Distillation,  188. 
Divisibility,  16. 
Double  refraction,  70. 
Ductility,  19. 

E 

Ear,  The,  142. 
Ear  of  Dionysius,  145. 
Ear-trumpet,  128. 
Echoes,  129. 
Elasticity,  20. 
Electric  battery,  233. 
44       chime,  224. 
light,  236. 
44       potential,  233. 
44       telegraph,  242. 
44       whirl,  227. 
Electrical-machine,  220,  221. 
Electricity,  211. 

44          Frictional,  215. 
Galvanic,  231. 
44          Magnetic,  211. 
Electrodes,  232. 
Electrophorus,  218. 
Electro-gilding,  239. 
44       magnetism,  239. 

magnets,  240. 

"       negative  and   positive   sub- 
stances, 237. 
44       plating,  238. 
Electrolysis,  237. 
Electroscope,  216. 
Energy,  36. 
Equilibrium,  56. 
Eustachian  tube,  143. 
Evaporation,  191. 
Expansion,  186. 
Eye,  The,  I7S. 


Falling  bodies,  54. 

Faraday,  40,  207,  223. 

Far-sightedness  (hyperopia\  177. 

Filtering,  17. 

Fire-engine,  112. 

Fish,  96. 

Flames,  Sensitive,  140. 

Singing,  141. 
Floating  bodies,  95. 
Fly-wheel,  78. 


Focus,  159. 
Fogs,  198. 
Force,  27. 

44      pump,  112. 

44      Centrifugal,  33. 

"      Centripetal,  33. 

44      Molecular,  43. 
Forces,  Parallelogram  of,  31. 

14       Resolution  of,  31. 
Foucault,  62. 
Fountains,  89. 
Franklin,  228. 
Fraunhofer's  lines,  165. 
Freezing  mixture,  188. 

44        of  water,  191,  202. 
Friction,  27. 
Frost,  198. 
Fulcrum,  69. 

G 

Galvanometer,  240.    ^ 
Galileo,  40,  65,  80,  119,  146. 
Gases,  43. 

44      Adhesion  of,  49,  115. 

44      Buoyancy  of,  108. 

41      Compressibility  of,  21,  105. 

41      Diffusion  of,  48. 

14      Elasticity  of,  zr,  «,,. 

u      Osmose  of,  51. 

41      Pressure  of,  106. 
Geissler's  tubes,  247. 
Glassware,  46. 
Gold-leaf,  Making  of,  20. 
Governor,  The,  195. 
Gravitation,  52. 
Gravity,  52. 

44       Centre  of,  56. 

41       Specific,  92. 
Guericke,  107,  no. 
Gulf  stream,  201. 
Gyroscope,  35. 

H 

Halos,  167. 
Hardness,  21. 
Harmonics,  138. 
Hay-scales,  72. 
Heat,  183. 

44      affected  by  rarefaction,  1971 
Absorption  of,  194. 
Conduction  of,  192. 
44      Convection  of,  193. 
Expansion  by,  186. 
44      Latent,  187. 
44      Luminous,  183. 


INDEX. 


308 


Heat,  Mechanical  equivalent  ot, 

"      Radiation  of,  193. 

"      Reflection  of,  194. 

"     Specific,  186. 

44     Theory  of,  184. 
Heating  by  steam,  206. 
Helix,  241. 
Helmholtz,  143,  146. 
Hero's  fountain,  106. 
Holtz's  machine,  221. 
Horse-power,  A,  79. 
Huygens,  40,  60, 180. 
Hydraulics,  97. 
Hydraulic  ram,  114. 
Hydrometer,  94. 
Hydrostatics,  83. 
Hydrostatic  bellows,  88. 
paradox,  89. 
press,  85. 

I 

Ice-crystals,  45,  200. 
Iceland  spar,  170. 
Impenetrability,  16. 
Inclined  plane,  74. 
Indestructibility,  18. 
Index  of  refraction,  158, 
Induction,  212,  223. 
Inertia,  18,  29. 
Insulators,  219. 
Interference,  103, 134. 
Isochronous,  59. 

J 

Joule's  law,  185. 

K 

Kaleidoscope,  153. 
Kite,  32- 

L 

Leconte,  207. 
Lenses,  159. 

Land-and-sea  breeze,  200. 
Lever,  69. 
Leyden  jar,  225. 
Light,  149. 

Composition  of,  163. 

Diffraction  of,  168. 

Interference  of,  168. 

Laws  of,  150. 

Polarized,  170. 

Reflection  of,  151. 

Refraction  of,  157. 

Theory  of,  150. 

Total  reflection  of,  156. 


Tjgm,  V  eiocity  ot,  150, 

"      Waves  of,  151. 
Lightning,  227. 
Liquids,  Buoyancy  of,  95. 

Cohesion  of,  43,  44. 

Compressibility  of,  21. 

Diffusion  of,  49. 

Elasticity  of,  21. 

Osmose  of,  50. 

Pressure  of,  86. 

Specific  gravity  of,  94. 

tend  to  spheres,  44. 
Liquefaction,  187. 

"  of  gases,  «o6. 

Lissajous,  146. 
Locke,  206. 

M 

Machinery,  69. 

Magdeburg  hemispheres,  107. 
Magic  lantern,  174. 
Magnetic  curves,  211. 
Magnetism,  211. 
Magneto-electricity,  211. 
Magnets,  211. 
Magnitude,  15. 
Malleability,  19. 
Mariotte's  law,  109. 


Measures,  Standards  of,  23*  ga. 
Mechanical  powers,  69. 
Mechanics,  Principle  of,  ocj. 
Meteorology,  197. 
Microphone,  248. 
Microscopes,  171. 
Mirage,  162. 
Mirrors,  151. 
Molecules,  13. 
Molecular  forces,  43. 
Moment  of  a  force,  70. 
Momentum,  27. 
Motion,  27. 

Compound,  30. 

Circular,  33. 

Laws  of,  28. 

Perpetual,  78. 

Reflection  of,  35. 

Resistance  to,  27. 
Multiple  images,  153. 
Music,  131. 
Musical  scale,  139. 

N 

Near-sightedness 
Needle,  Astatic,  341- 


304 


Needle,  Magnetic,  212. 

"       Dipping,  215. 
Newton,  40,  65,  i8a 
Newton's  rings,  168. 
Nodal  lines,  137. 
Nodes,  136. 
Noise,  131. 
Northern  lights,  228. 


Ocean  currents,  201. 
Octave,  133. 
Opera-glass,  173. 
Optics,  149. 

Optical  instruments,  171. 
Organ  pipes,  140. 
Oscillation,  Centre  of,  60. 
Osmose  of  gases,  51. 

liquids,  50. 
Overtones,  138. 


Page's  rotating  machine,  241. 

Pascal,  85,  120. 

Pendulum,  59. 

Percussion,  Centre  of,  ox. 

Perpetual  motion,  78. 

Phonograph,  141. 

Pinion,  73. 

Pisa,  Tower  of,  58. 

Pitch,  131. 

Platinum  wire,  19. 

Plato,  22. 

Plumb-line,  54. 

Pneumatics,  103. 

Pneumatic  inkstand,  114 

Polarization  of  light,  170. 

electricity,  213,  219. 
Porosity,  16. 
Pressure  of  air,  106. 
Prince  Rupert's  drop,  46. 
Prisms,  158. 
Pulley,  76. 
Pumps,  in. 

Air,  103. 

Force,  m. 

Lifting,  xi2. 

Sprengel's  air,  118. 
Pythagoras,  146. 


Radiometer,  19*. 
Rain,  199. 

165. 


Reaction,  30. 
Reaction-wheel,  100. 
Reflected  motion,  35. 
Reflection,  Total,  156.  ^ 
Refraction,  Index  of,  15* 
Regelation,  202. 
Relay,  245. 
Resonance,  140. 
Rivers,  98. 

Ruhmkorff 's  coil,  247. 
Ruinford,  Count,  207. 
Rupert's  drop,  46. 


St.  Elmo's  fire,  229. 
Screw,  75. 

Sensitive  flames,  140. 
Ship,  Sailing  of,  37. 
Singing  flames,  141. 
Siphon,  112,  119. 
Siren,  131. 
Size,  15,  21. 
Snow,  199. 
Solution,  48. 
Sonometer,  135. 
Sound,  121. 

"      Intensity  of,  137. 
in  a  vacuum,  125. 
Interference  of,  1340 
Production  of,  123. 
"      Reflection  of,  129. 
"       Refraction  of,  128. 
u       Superpositon  of,  133, 
u       Transmission  of,  1*4, 
"      Velocity  of,  126. 
Sounding-boards,  135. 
Sound  waves,  124. 
Speaking  tubes,  128. 

"        trumpet,  128. 
Specific  gravity,  92. 

"        flask,  94. 
Spectroscope,  165. 
Spectrum  analysis,  165. 

"         Solar,  163. 
Spherical  aberration,  161. 
Spheroidal  state,  192. 


"       engine,  195. 
Steelyard,  71. 
Stereopticon,  174. 
Stereoscope,  174. 
Stringed  instruments,  134, 
Substance,  13. 
Surface  tension,  47. 


INDEX. 


305 


tacking,  33. 
Tackle-block,  78. 
Telegraph,  242. 
Telephone,  247  (see  note,  127). 
Telescope,  172. 
Temperature,  186. 
Tempering,  46. 
Tenacity,  20. 
Thermal-electricity,  249. 
Thermometers,  186. 
Thunder,  227. 
Torricelli,  119. 
Torsion  balance,  21. 
Total  reflection,  156. 
Trade-wind,  200. 
Tubes,  98. 
Turning  effort,  70. 
Turbine  wheel,  100. 
Tyndall,  146,  207. 


Vaporization,  188. 

Velocity,  27. 

Velocity  of  electricity,  229. 

heat,  183. 

light,  150. 

sound,  126. 
Vertical,  54. 
Vibration,  A,  134. 
Vibrations  of  air,  124. 

cords,  134. 


Vibrations  of  ether,  151. 

pendulum,  59. 
"          Sympathetic,  140. 
Virtual  velocity,  80. 
Visual  angle,  149. 
Vocal  Memnon,  145. 
Voltaic  arch,  236. 

"       battery,  233,  252. 

•»       electricity,  231. 

"       pair,  The,  232. 

W 

Watches,  66. 
Water,  201. 

"       barometer,  iia 
level,  91. 

"       wheels,  99. 
Waves,  101,  133. 
Wave  motion,  101. 
Wedge,  76. 
Welding,  44. 
Weight,  52,  53. 
Wells,  89. 

Wheel  and  axle.  72. 
Wheel-work,  73. 
Whirl-i-gig,  100. 
Wild's  machine,  2/16. 
Winds,  200. 
Wind  instruments,  140 

Y 

Young,  180. 


' 


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Centrifugal  Hoop. 
Inertia  Apparatus. 
Capillary  Attraction. 
Collision  Balls  (ebonized  base,  7  balls). 
Prince  Rupert's  Drops,  l  doz. 
Gyroscope. 
Pair  Cohesion  Plates. 
Double    (Gravity)  Cone,  that  runs  up 
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HYDROSTATICS. 

Equilibrium  Tubes  (full  set). 

Bottle  Imp  and  Jar. 

Force  Pump,  Glass. 

Lift  Pump,  Glass. 

Siphon. 

Water  Hammer. 

Hydrometer,  to  show  specific  gravity  of 

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Hydrometer  Jar. 


PNEUMATICS. 

Air  Pump,  valveless. 

Gallon  Receiver  (swelled). 

Hand  and  Bladder  Glass. 

Torricellian  (Barometer)  Tube  with 
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Mercury  Shower. 

Magdeburg  Hemispheres. 

Clamp  for  Air  Pump. 

Can  of  Oil. 

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OPTICS. 

Prism,  4  in. 

Demonstration  Lenses  (set  Six). 
Microscope,  compound. 
Concave  and  Convex  Mirrors. 
Iceland  Spar  (section). 


ACOUSTICS. 
Telephone,    Acoustic,    (complete   with 

fixtures). 
Tuning  Fork. 
Brass  Plats  and  holder,  for  acoustic,  or 

Chladni,  figures. 
Bow,  to  cause  Vibration. 
Diapason,  small,  for  tracing  Vibrations 

011  smoked  glass. 

LIGHT   AND    HEAT. 
Newton's  Disc 
Palm  Glass,  large. 
Radiometer,  Crookes's. 
Compound  Bar,  to  show  expansion. 
Spirit  Lamp,  Brass. 
Air  Thermometer. 

MAGNETISM  AND  GALVANISM. 
Bichromate   of   Potash   Battery,  quart 

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Bar  Magnet.  0  in. 

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Geissler   Tube,  8   in.   long,   containing 

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Geissler  Tube,  8   in.   long,  containing 

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MOTION,    FORCE,   AND  PROPERTIES 
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Centrifugal  Hoop. 
Inertia  Apparatus. 
Capillary  Attraction  Tubes. 
Collision  Balls  (5  Balls). 
1  Doz.  Prince  Rupert's  Drops. 

HYDROSTATICS. 

Equilibrium  Tubes  (4  tubes). 

Bottle  Imp  and  Jar. 

Glass  Force  Pump. 

Glass  Lift  Pump. 

Glass  Siphon. 

Water  Hammer. 

Hydrometer,  to  show  specific  gravity  of 

Liquids. 
Hydrometer  Jar. 

PNEUMATICS. 
Air  Pump,  improved. 
Receiver,  gallon. 
Hand  and  Bladder  Glass. 
Torricellian     (Barometer)    Tube    with 

Pipette,  for  filling  with  Mercury. 
Freezing   Apparatus,  used  with  gallon 

receiver. 
Fountain    in   Vacuo,  and   Guinea  and 

Feather  Tube,  combined. 
Magdeburg  Hemispheres. 
Sheet  Rubber,  square  foot. 


HEAT. 

Compound  Bar,  to  show  Expansion. 
Palm  Glass,  large. 
Air  Thermometer. 
Alcohol  Lamp,  brass. 

OPTICS. 
Prisms,  3  in. 

Demonstration  Lenses,  set  of  6. 
Compound  Microscope. 
Concave  and  Convex  Mirrors. 

ELECTRICITY. 

Toepler-Holtz  Electrical  Machine,No.  1. 
Leyden  Jar,  qt. 
Discharger. 
Electric  Bell  Chime. 
Plates  for  Images. 
Pair  Pith  Images. 
Geissler  Tube,  8  in. 
Electrical  Machine  Attachment. 
Brass  Chain. 
Insulated  Wire. 

MAGNETISM   AND   GALVANISM. 
Bichromate  of  Potash  Battery. 
Induction  Coil. 
Bar  Magnet. 
Electro  Magnet,  supporting  from  10  to 

20  Ibs. 

Magnetic  Needle. 
Horse-shoe  Magnet,  5X  in. 


The  regular  price  of  above  articles  boxed  is  $122.15. 
We  are  able,  by  preparing  the  sets  in  large  quantities,  to 
give  purchasers  a  greater  reduction  than  if  they  should 
select  a  list  of  their  own. 

For  schools  not  limited  to  an  expenditure  of  $100,  set 
No.  2  covers  a  much  larger  field. 

ADDRESS:         A.  S.  BARNES  &  CO., 

NEW   YORK,  or  CHICAGO. 


A    NEW    AND    ENLARGED.    SET    OF 

CHEMICAL  APPARATUS. 

Prepared  expressly  for  the  performance  of  the  experiments 
in  the  new   edition  of  Steele's  Fourteen  Weeks  in  Chemistry. 


PRICE,   $30, 


Acid,  Sulphuric. 
'      Nitric. 
4     Hydrochloric. 
'     Arsenious. 
'     Oxalic. 
'  .  Tartaric. 
Ammonium  Chloride. 

Nitrate. 

"         Sulphide. 
Ammonia  Water. 
Antimony. 
Alcohol. 

Barium  Chloride. 
"      Nitrate. 
Bone  Black. 

Cobalt  Nitrate  (solution). 
Calcium  Fluoride. 
"        Sulphate. 
Copper. 

Carbon  Disulphide. 
Ferrous  Sulphide. 
"      Sulphate. 
Gun  Cotton. 
Iodine. 
Lead  Acetate. 

"    Monoxide. 
Litmus  (best).  • 

Magnesium  Ribbon. 
Manganese  Dioxide. 
Mercury. 
Mercuric  Chloride. 

"         Oxide. 
Nut  Galls  (powdered) 
Potassium  Ferricyanide. 
Iodide. 

Permanganate. 
Bichromate. 
Phosphorus. 
Potassium  (Metallic.) 
Chlorate. 


Potassium  Hydrate. 

Nitrate. 

Chromate. 

Cyanide. 

Ferrocyanide 
Sulphuric  Ether. 
Sodium  (Metallic). 
Biborate. 
"      Carbonate. 
"      Sulphate. 
Strontium  Chloride. 

Nitrate. 
Sulphur. 
Silver  Nitrate. 
Blowpipe. 
Deflagrating  Spoon. 
Evaporating  Dishes. 
Evolution  Flask. 
File  (triangular). 
Funnel. 

Filtering  Paper. 
Florence  Flask. 
Graduate. 

Glass  Tubing,  assorted. 
Gas  Bag  with  Stop  Cock. 
Lead  Dish. 
Metric  Graduate. 
Nest  of  Crucibles. 
Pneumatic  Trough. 
Retort. 

Retort  Stand  (3  Rings). 
Rubber  Tubing. 
Scales  and  Weights. 
Spirit  Lamp. 
Test  Tubes. 
Wedgewood  Mortar. 
Wire  Gauze. 
Wooden  Retort  Stand,  with 

cork  nhieldg 


This  Set  is  adequate  for  the  performance   of  all  the 
experiments  in  any  ordinary  text  book. 


CHEMICAL  APPARATUS. 


This  Set  is  adequate  for  the  performance  of  the 
leading  experiments  in  any  text-book.  We  would  call  special 
attention  to  the  extremely  low  price  at  which  it  is  offered. 

PRICE,   $15. 


Alcohol. 

Acid,  Sulphuric. 

"      Nitric. 

"      Hydrochloric. 

"      Arsenious 

"      Oxalic. 

"      Tartaric. 
Ammonium  Chloride. 

Nitrate. 

"  Sulphide. 

Ammonia  Water. 
Antimony  (Metallic). 
Barium  Chloride. 

Nitrate. 
Bone  Black. 
Calcium  Fluoride. 
"        Sulphate. 
Copper  Sulphate. 
Carbon  Disulphide. 
Ether. 

Ferrous  Sulphide. 
"        Sulphate. 
Gun  Cotton. 
Iodine. 

Lead  Acetate. 
Litmus. 
Mercury. 
Magnesium  Ribbon. 

"      Monoxide. 
Manganese  Dioxide. 
Nut  Galls. 
Phosphorus. 


Potassium. 

"        Ferrooyanide. 

"        Chlorate. 

"        Hydrate. 
Nitrate. 

"        Bichromate. 
Strontium  Chloride. 
Nitrate. 
Sulphur, 
Silver  Nitrate. 
Sodium  (Metallic.) 
"        Biborate. 
"        Carbonate. 
"        Sulphate. 
Blowpipe. 

Deflagrating  Spoon. 
Evaporatiug  Dish. 
Evolution  Flask. 
Filters. 
File. 
Funnel. 
Graduate. 
Glass  Tubing. 
Lead  Dish. 
Nest  of  Crucible*. 
Retort. 

Rubber  Tubing. 
Spirit  Lamp. 
Test  Tubes. 
Tripod. 
Wedgewood  Mortar. 


We  would  recommend  the  above  to  such  schools  as 
have  not  the  means  to  purchase  the  more  comprehensive  set, 


R  A  * 

OF  THE 

UNIVERSITY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 


J\)t  3° 

AUG  28  1916 
APR  15  1918 
MAY  111918 


JAN  2  3  1985     % 


QLC 16 1969  2 
JA<  1  6 1970 

fit 


161970 

«8CDLD'IJAY 


CIRAPR26  1985 


1870-3DPM  19' 


